Separate the theorydb utilities into the data package

[scarlehoff]

This is innocuous since the theorydb utils just define which entries the theory accepts.

This will make it easier for other programs (e.g., pineko) to use theories from here. The global definition of the theory can be done in the _TheoryCard dataclass. At the moment only the keys for NNPDF are there and they are all mandatory (even if it is not true).

[felixhekhorn]

Do you want to do a further rename here theorydbutils -> theory?

[scarlehoff]

No, at the moment externally these are just the utilities.
The proper theory.py can be born once someone creates the proper Theory class (at the moment there is a mock dataclass where I just declared all the fields that were in the sqlite database)

[RoyStegeman]

Other than the comment about the docs, the rest looks fine. Thanks!