Add new POTCAR validation to VASP validation plus tests

[esoteric-ephemera]

Building on PR #874, rewrote _potcar_hash_check in emmet.core.vasp.validation to use the new POTCAR summary_stats method in pymatgen. This method is less brittle than the older md5 hashing scheme.

Also updated emmet.builders.vasp.task_validator to use the _summary_stats attribute of the PotcarSingle class rather than the md5 hash in creating the potcar_hash field.

Also added two tests:

• One for the new POTCAR check in tests/vasp/test_vasp.py
• And, per a request from @janosh that I missed in PR #874 (sorry!), a test for emmet.core.vasp.calculation.PotcarSpec to ensure that the summary_stats field is read

[codecov-commenter]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (f4d3132) 91.29% compared to head (763e2e8) 78.89%.

Files	Patch %	Lines
emmet-core/emmet/core/vasp/validation.py	0.00%	16 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #892 +/- ## =========================================== - Coverage 91.29% 78.89% -12.41% =========================================== Files 138 75 -63 Lines 12779 4231 -8548 =========================================== - Hits 11667 3338 -8329 + Misses 1112 893 -219 ```

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