Ensure all MP2020 corrections have unique names; tweak oxi state detection

[rkingsbury]

Ensures that energy corrections applied to each anion have unique names (e.g., N vs. Cl vs. Br).

If this is not the case, then in a compound with multiple corrected anions like C2NCl, Compatibility will apply a correction to the first anion, then think the second is a duplicate, warning

warnings.warn(
2021-05-01T11:22:13.8878461Z /users/home/ghrunner/actions-runner/_work/_tool/Python/3.8.7/x64/lib/python3.8/site-packages/pymatgen/entries/compatibility.py:579: UserWarning: Entry mp-567221 already has an energy adjustment called MP2020 anion correction, but its value differs from the value of -0.718 calculated here. This Entry will be discarded.

Also makes some tweaks to oxidation state detection to work better with the Builders.

[coveralls]

Coverage decreased (-0.7%) to 83.008% when pulling c637790 on rkingsbury:default_mp2020 into 12d6e2b on materialsproject:master.