T24 and V24 lack of contribution of charm

Created by: msyan

I noticed that when defining the evolution basis there seems to be no charm contributions in T24 and V24.

'T24' : {'u': 1, 'ubar': 1, 'd': 1, 'dbar': 1, 's': 1, 'sbar': 1, 'b':-4, 'bbar':-4}, 'V24' : {'u': 1, 'ubar':-1, 'd': 1, 'dbar':-1, 's': 1, 'sbar':-1, 'b':-4, 'bbar':+4},

Line 494 of https://github.com/NNPDF/nnpdf/blob/master/validphys2/src/validphys/pdfbases.py