NEW Restructure python data IO/actions

Created by: wilsonmr

supercedes #1080 but I left that open just in case.

here I implemented: https://github.com/NNPDF/nnpdf/pull/1080#issuecomment-775878619

in addition to that I decided to just make the functions which built covmats into actions which meant we could get rid of unneccessary rubbish, in a similar manner to @siranipour's idea with #866 (simply changing the signature). I may have broken something along the way.

I think this is a good balance between not having unneccessary subdir but also it's slightly clearer where things should be