move `separate_multiplicative` key outside `sampling` key in runcard
Created by: RoyStegeman
This fixes vp-setupfit since there separate_multiplicative
was not parsed from the sampling
key and thus it was not found. This was broken in https://github.com/NNPDF/nnpdf/pull/2107
This also resolves #2106 (closed) by removing the sampling namespace all together, though since we never use it we may also want to drop the separate_multiplicative
keyword all together.