Add a basis with c & cbar

Created by: scarlehoff

Which also requires ensuring that the rotation to the fktable basis and sum rules work.

• Add the c-cbar basis
• FKTable rotation (see https://github.com/NNPDF/nnpdf/issues/1627#issuecomment-1311600732)
• Sum rules
• Check that old FK Tables are not optimizing out the ccbar asymmetry
• Check that pineappl FK Tables are not optimizing out the ccbar asymmetry

@felixhekhorn I added you to this issue just because of the last point ofc, let us now and if it is we just do a theory 4XY with the asymmetry.

Although I wonder whether the fktables in theory 200 would allow for it (I'll have a look at that)