Table header for `procs_chi2_table` says `groups` but content is actually dataset

Created by: scarlehoff

As mentioned in https://github.com/NNPDF/nnpdf/pull/1525#pullrequestreview-893579541_ the header of the table / name of the function / content is a bit confusing.

My favourite solution (I already talked with @andreab1997 about that) would be to change the table so that the grouping is another column so you get a table of datasets per group.