Revisit implementation of all DIS

[scarlehoff]

This issue is about revisiting the implementation of the NNPDF4.0 DIS data sets. There are two major classes of DIS data sets, that have been implemented over a period of twenty years, to different standards of quality checks:

• fixed target DIS data sets (SLAC, BCDMS, NMC, EMC, NuTeV, CHORUS), which are the oldest;
• collider data sets (HERA H1 and ZEUS combined), which are the newest.

These data sets have been ported from the old commondata to the new commondata format by @scarlehoff, assuming that the old commondata implementation was correct. This issue is about checking that the old commondata implementation was actually correct. We will do so by re-implementing the data from scratch, using the information available from Hepdata, and implementing all the variants as need be.