Extend DFT information

[ndaelman-hu]

Currently, only a handful of hybrid DFs currently know their default exchange factor mixing.
This coverage should be extended, preferably without hard-coding the defaults.

Alternative solutions may include using the libxc Python module, or producing an artifact from our own graph database.
In case of the latter, we should also consider whether registering the default is appropriate for the normalizer, or should happen at a higher level.

[JosePizarro3]

One thing I was thinking after #105: now, xc_functionals is a sub-section of DFT, but with the new contributions concept, I think this should be restructured, i.e., delete xc_functionals sub-section while keeping the class but inheriting from BaseModelMethod instead of ArchiveSection.

Does this makes sense for you?

[JosePizarro3]

It is also part of the Assignments of the tutorial: https://fairmat-nfdi.github.io/fairmat-tutorial-14-computational-plugins/nomad_simulations/#modelmethod Assignment 2.3