updates(model_splitter.py): update UnstructuredGrid support

[jlarsen-usgs]

• force remapping to be calculated from iac, ja array
• add 0 digit padding to file names
• update UnstructuredGrid, add cell2d input parameter and property method
• add clean_iverts() to UnstructuredGrid to remove duplicated vertex and invert information
• update testing for new features

[codecov]

Codecov Report

Attention: Patch coverage is 72.50000% with 33 lines in your changes missing coverage. Please review.

Project coverage is 74.4%. Comparing base (559ae74) to head (2cd8fd8).
Report is 8 commits behind head on develop.

Files	Patch %	Lines
flopy/discretization/unstructuredgrid.py	50.8%	29 Missing ⚠️
flopy/mf6/utils/model_splitter.py	92.4%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2292     +/-   ##
=========================================
+ Coverage     73.9%   74.4%   +0.4%     
=========================================
  Files          294     294             
  Lines        59394   59486     +92     
=========================================
+ Hits         43932   44268    +336     
+ Misses       15462   15218    -244     

Files	Coverage Δ
flopy/mf6/mfmodel.py	82.8% <100.0%> (-0.2%)	⬇️
flopy/utils/rasters.py	74.2% <100.0%> (+0.3%)	⬆️
flopy/mf6/utils/model_splitter.py	70.5% <92.4%> (+0.7%)	⬆️
flopy/discretization/unstructuredgrid.py	80.8% <50.8%> (-3.8%)	⬇️

... and 10 files with indirect coverage changes

[jdhughes-usgs]

@jlarsen-usgs is this ready to go in or do you have more to do?

[jdhughes-usgs]

@jlarsen thanks for the model name padding.

[jlarsen-usgs]

@jdhughes-usgs
It's ready to go in. I'll merge it now.