Docs update, forcefield elastic convenience maker, forcefield enum hydration

[esoteric-ephemera]

Three smaller changes:

• Update VASP EOS section of the docs to explain the makers / sets there, close issue #1071
• Add a convenience constructor method, from_force_field_name to ElasticMaker as suggested by @utf in #999
• Allow the forcefield name enum MLFF to permit str(MLFF.<member>) as valid input. This should hopefully resolve some issues noted by @tjaffrel when running forcefield jobs from fireworks

[JaGeo]

@esoteric-ephemera anything else to add? If not, i am going to merge.

[esoteric-ephemera]

Should be ready to merge, thanks @JaGeo!

[JaGeo]

@esoteric-ephemera thank you so much!