Write a validphys function to process fktables

Created by: Zaharid

Should have a boolean mask to select the flavours, the xgrid and sigma.

sigma should have the following dimensions for hadronic:

ndata x nchannels x nx x nx

Where nchannels loops over the active partonic channels, which are encoded in the file.

For DIS, it is:

ndata x nflavours x nx