Parsing Hubbard parameters

The Hubbard parameters (as listed in the metainfo) should be added to the parser

Note on the schema: dft_plus_u.functional (as defined), is superfluous. What would be more sensible is renaming it to dft_plus_u.method/dft_plus_u.approach for catching the physics model, i.e. Lichtenstein, Dudarev, etc.

Some VASP examples:

• zRE1NkTdewiRS3vahFQQ5og0RTbz (and all cerium oxide calculations from 10.17172/NOMAD/2020.02.18-3)
• ZaS0VzyvW0tAGXN4HEwZ9w_0Vo3H (and most oxides from AFLOW computed at LDA, GGA, or mGGA level)