Create a core Theory object

[scarlehoff]

At the moment theoryid.get_decription() returns simply a dictionary. It would be useful to obtain a dataclass instead from which some derived objects can be calculated like the fitting nf.

Creating an object for the theory is also a good way of defining what a theory is prior to #1705.

[scarlehoff]

Pineko creates the theory:

• grids (yadism/madgraph/etc)
• grids -> fktables through eko
• eko.tar
• eko_qed.tar (if needed)

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Once all of above is done, the nnpdf can be "upgraded" to said new theory.

[scarlehoff]

Moved the issue to NNPDF/pineko#103