Add vp parser for new commondata format

[Zaharid]

We would like to take the input in #1500 and write a parser that results in essentially coredata.Commondata,

nnpdf/validphys2/src/validphys/coredata.py

Line 123 in ab7f053

class CommonData:

with a few adjustments such as supporting n kinematics.

That should come with the accompanying machinery for Loader. We are going to have some more context dependent options, such as the relevant variant that gets selected.

[scarlehoff]

Superseded by #1709