Parsing: POTCAR versions (formerly: VASP RunType is missing PBE_64/LDA_64)

[tsmathis]

previously brought up as issue #1101

quoting @esoteric-ephemera:

• Creating a pymatgen Potcar from a file in a library, we can store the version (PBE_54, PBE_64,...) because the library is structured to reflect these versions
• Creating a Potcar from a calculation directory where it's just a single POTCAR file, like we would when parsing TaskDoc.from_directory: We can't store a version because multiple POTCAR releases might have the same contents as the POTCAR file
  Solution might just be providing a list of possible POTCAR versions

no planned implementation yet

Open to PRs + suggestions