[WIP] Add new MP input sets

[esoteric-ephemera]

This PR adds new MP input sets that resulted from a benchmarking effort over the past year. For the benchmarks, refer to:

• https://github.com/materialsproject/foundation/pull/26
• Reference living document

Critical updates include:

• POTCARs
• KSPACING adjustment by bandgap
• LREAL = False
• LMAXMIX = 6

The updated KSPACING function is slightly different from what was discussed with the MP foundation initially. This is to ensure that calculations with the highest $k$-point density (KSPACING = 0.22) would not be deprecated by this PR. Previously I recommended that the minimum KSPACING = 0.2, but this is effectively the same as 0.22.

Leaving this up as a WIP pending further community discussion and writing tests