move `separate_multiplicative` key outside `sampling` key in runcard

Emanuele Roberto Nocerarequested to merge
fix_sep_mult_key into master

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.

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