fix(mfmodel): fix budgetkey for transport models

[wpbonelli]

• add budgetkey param to MF6Output and pass it down to MF6ListBudget in MF6Output.__list()
• update MFModel.output to set budgetkey to "MASS..." instead of volume for transport models
• fix bug: Budget from transport model listing file is not returned #2087

[wpbonelli]

this gives the following for synthetic valley

(Pdb) gwt.output
MF6Output Class for oc
Available output methods include:
.list()
.zonebudget()
.budget()
.budgetcsv()
.concentration()
(Pdb) gwt.output.list().budgetkey
'MASS BUDGET FOR ENTIRE MODEL'
(Pdb) gwt.output.list().get_dataframes()
(                            STORAGE-AQUEOUS_IN  WEL_IN  DRN_IN  RCHA_IN  EVTA_IN  SFR_IN      LAK_IN  ...  RCHA_OUT  EVTA_OUT    SFR_OUT  LAK_OUT   TOTAL_OUT    IN-OUT  PERCENT_DISCREPANCY
1970-07-02 14:52:47.578125            0.035934     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    0.4838   0.000010      0.0  840.631592 -1.117300                -0.13
1971-01-01 06:00:00.000000            0.019396     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    0.8427   0.000069      0.0  840.097412 -0.599700                -0.07
1971-07-02 21:07:12.421875            0.011008     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.0972   0.000257      0.0  839.765930 -0.276500                -0.03
1972-01-01 12:00:00.000000            0.006263     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.2850   0.000853      0.0  839.665771 -0.181200                -0.02
1972-07-02 02:52:47.578125            0.003998     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.4301   0.002386      0.0  839.606506 -0.124200                -0.01
1972-12-31 19:12:04.218750            0.002891     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.5467   0.005770      0.0  839.569702 -0.088412                -0.01
1973-07-02 09:36:02.109375            0.002316     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.6439   0.012399      0.0  839.544678 -0.064027                -0.01
1974-01-01 00:00:00.000000            0.001858     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.7273   0.024163      0.0  839.526672 -0.046517                -0.01
1974-07-02 14:23:57.890625            0.001564     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.8007   0.043433      0.0  839.513000 -0.033049                -0.00
1975-01-01 04:47:55.781250            0.001370     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.8666   0.072991      0.0  839.501404 -0.021688                -0.00
1975-07-02 21:36:02.109375            0.001200     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.9269   0.115900      0.0  839.491394 -0.011845                -0.00
1976-01-01 12:00:00.000000            0.001042     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    1.9830   0.175400      0.0  839.482605 -0.003209                -0.00
1976-07-02 02:24:08.437500            0.000923     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.0359   0.254600      0.0  839.474609  0.004710                 0.00
1976-12-31 19:12:04.218750            0.000804     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.0862   0.356500      0.0  839.467285  0.011868                 0.00
1977-07-02 09:35:51.562500            0.000716     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.1346   0.483800      0.0  839.460693  0.018408                 0.00
1978-01-01 00:00:00.000000            0.000646     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.1814   0.638700      0.0  839.454712  0.024286                 0.00
1978-07-02 14:24:08.437500            0.000597     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.2269   0.823100      0.0  839.449524  0.029461                 0.00
1979-01-01 04:47:55.781250            0.000552     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.2713   1.038400      0.0  839.445129  0.033844                 0.00
1979-07-02 21:35:51.562500            0.000506     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.3148   1.285600      0.0  839.441101  0.037752                 0.00
1980-01-01 12:00:00.000000            0.000449     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.3575   1.565500      0.0  839.437805  0.041044                 0.00
1980-07-02 02:24:08.437500            0.000375     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.3995   1.878700      0.0  839.434998  0.043721                 0.01
1980-12-31 19:12:04.218750            0.000319     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.4407   2.225100      0.0  839.432800  0.045889                 0.01
1981-07-02 09:35:51.562500            0.000288     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.4813   2.604500      0.0  839.431091  0.047529                 0.01
1982-01-01 00:00:00.000000            0.000267     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.5212   3.016300      0.0  839.429871  0.048740                 0.01
1982-07-02 14:24:08.437500            0.000240     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.5605   3.459400      0.0  839.429688  0.048932                 0.01
1983-01-01 04:48:16.875000            0.000216     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.5990   3.932600      0.0  839.429321  0.049240                 0.01
1983-07-02 21:35:51.562500            0.000192     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.6368   4.434600      0.0  839.429993  0.048523                 0.01
1984-01-01 12:00:00.000000            0.000171     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.6739   4.964000      0.0  839.430176  0.048324                 0.01
1984-07-02 02:24:08.437500            0.000153     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.7103   5.519400      0.0  839.430725  0.047797                 0.01
1984-12-31 19:11:43.125000            0.000137     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.7458   6.099400      0.0  839.431885  0.046576                 0.01
1985-07-02 09:35:51.562500            0.000116     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.7805   6.702500      0.0  839.434326  0.044159                 0.01
1986-01-01 00:00:00.000000            0.000090     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.8145   7.327200      0.0  839.435913  0.042588                 0.01
1986-07-02 14:24:08.437500            0.000062     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.8475   7.971700      0.0  839.437012  0.041441                 0.00
1987-01-01 04:48:16.875000            0.000041     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.8798   8.634000      0.0  839.438171  0.040226                 0.00
1987-07-02 21:35:51.562500            0.000025     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.9111   9.312700      0.0  839.439392  0.039006                 0.00
1988-01-01 12:00:00.000000            0.000015     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.9416  10.006000      0.0  839.440796  0.037609                 0.00
1988-07-02 02:24:08.437500            0.000013     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    2.9712  10.712400      0.0  839.442078  0.036309                 0.00
1988-12-31 19:11:43.125000            0.000016     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.0000  11.430600      0.0  839.443176  0.035148                 0.00
1989-07-02 09:35:51.562500            0.000019     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.0279  12.159000      0.0  839.444275  0.034114                 0.00
1990-01-01 00:00:00.000000            0.000021     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.0550  12.896200      0.0  839.445679  0.032653                 0.00
1990-07-02 14:24:08.437500            0.000021     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.0812  13.640400      0.0  839.447327  0.031093                 0.00
1991-01-01 04:48:16.875000            0.000022     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.1065  14.390100      0.0  839.448730  0.029730                 0.00
1991-07-02 21:35:51.562500            0.000023     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.1311  15.143600      0.0  839.450012  0.028402                 0.00
1992-01-01 12:00:00.000000            0.000030     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.1548  15.899200      0.0  839.451111  0.027294                 0.00
1992-07-02 02:23:26.250000            0.000013     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.1766  16.461000      0.0  838.080078  1.398300                 0.17
1992-12-31 19:11:43.125000            0.000044     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.1980  17.059799      0.0  838.077820  1.400600                 0.17
1993-07-02 09:36:33.750000            0.000203     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.2188  17.686001      0.0  838.097290  1.381300                 0.16
1994-01-01 00:00:00.000000            0.000489     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.2391  18.336300      0.0  838.123108  1.355800                 0.16
1994-07-02 14:23:26.250000            0.000982     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.2588  19.006701      0.0  838.151672  1.327700                 0.16
1995-01-01 04:48:16.875000            0.001338     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.2779  19.693001      0.0  838.181274  1.298400                 0.15
1995-07-02 21:36:33.750000            0.001581     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.2964  20.390600      0.0  838.211426  1.268500                 0.15
1996-01-01 12:00:00.000000            0.001744     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.3142  21.094900      0.0  838.241699  1.238500                 0.15
1996-07-02 02:23:26.250000            0.001842     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.3315  21.801901      0.0  838.271606  1.208600                 0.14
1996-12-31 19:11:43.125000            0.001899     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.3482  22.507799      0.0  838.301270  1.179000                 0.14
1997-07-02 00:00:00.000000            0.001929     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.3643  23.209999      0.0  838.330872  1.149400                 0.14
1998-01-01 00:00:00.000000            0.001946     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.3798  23.905899      0.0  838.360474  1.119800                 0.13
1998-07-03 00:00:00.000000            0.001977     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.3947  24.593599      0.0  838.389526  1.090900                 0.13
1999-01-01 00:00:00.000000            0.001972     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.4092  25.271400      0.0  838.418091  1.062300                 0.13
1999-07-03 00:00:00.000000            0.001937     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.4231  25.938101      0.0  838.445984  1.034300                 0.12
2000-01-02 00:00:00.000000            0.001880     0.0     0.0      0.0      0.0     0.0  839.478394  ...       0.0    3.4365  26.592699      0.0  838.473328  1.006900                 0.12

[60 rows x 18 columns],                             STORAGE-AQUEOUS_IN  WEL_IN  DRN_IN  RCHA_IN  EVTA_IN  SFR_IN        LAK_IN  ...  RCHA_OUT      EVTA_OUT       SFR_OUT  LAK_OUT     TOTAL_OUT       IN-OUT  PERCENT_DISCREPANCY
1970-07-02 14:52:47.578125            6.562500     0.0     0.0      0.0      0.0     0.0  1.533097e+05  ...       0.0     88.362900      0.001884      0.0  1.535203e+05  -204.040207                -0.13
1971-01-01 06:00:0

[langevin-usgs]

I think this is fine for now, but I wonder if we need a more general way to set up these model-specific parameters. I think GWE probably has "ENERGY BUDGET FOR ENTIRE MODEL"? I'd say that we should override the output property for each flopy model object, but those classes are dynamically created. Looks like there is some model-specific code in MF6Output that would also benefit from generalization. Don't think we need to rethink this now, but something to keep in mind as the number of model types increases.