Substitute references to the yamldb for the new commondata

[scarlehoff]

This is mostly to track the new commondata PR in the nnpdf repo. The goal should be for the yamldb not to get to the public, otherwise it won't ever go away.

• Reader [WIP] Proposal for the common data reader nnpdf#1583
• DY [New commondata format] DY nnpdf#1610