Add vicker's hardness dataset

[ardunn]

Previous PR #765 was accidentally closed due to merge problems, but we are still waiting on @ziyan1996 @BrgochGroup to see to some questions about usage (i.e., if they will allow us to use and modify the data for redistribution with proper attribution) and data included:

1. Is it ok to upload a .json.gz version of this dataset to our figshare so there is a persistent resource for download?
2. Would it be possible to include any or all of the following as additional columns:

• Original source reference (doi and/or bibtex preferred)
• Spacegroup number or HM symbol
• Any other pertinent sample descriptors that might have been recorded in earlier versions of the dataset

Also go ahead and look over the metadata addition and make sure the name/description are correct and complete (and feel free to make any needed revisions).

Current data looks like this:

                composition  hardness  load
0     Ag0.05Gd0.048Pd0.902      1.810  0.49
1      Ag0.05Y0.048Pd0.902      1.640  0.49
2       Ag0.25Pb0.5Sb0.25Te     0.578  2.94
3       Al1.5Si1.5N2.5O1.5     15.030  0.98
4                 Al1.67B22    23.800  2.00
                     ...       ...   ...
1057                  ZrV2      3.600  0.98
1058                   ZrW2    11.100  0.98
1059                   OsB2    34.800  0.25
1060                   OsB2    27.000  0.49
1061                   OsB2    17.800  1.96

Once we figure out the above questions I'll upload the final version to figshare and the tests should pass.

Oops, apparently I already made this PR in #786