Make a light data-only package

Created by: scarlehoff

Given that some people have raised concern about the dependency hell that should be avoidable, it would be good to have a nnpdf-data package that includes only:

• data
• theory_db
• reader
• theory-reader

The only dependencies that would introduce are yaml, numpy, pandas and validobj. With both pandas and numpy being used only to actually load the data/uncertainties/etc but I think that it would be good for such a package to include that as well.