Add `resample_negative_pseudodata` to the `sampling` namespace

Created by: scarlehoff

Or make them both top-level variables. But it doesn't make sense to have them separated.

https://github.com/NNPDF/nnpdf/blob/e9f27634acecbd7629eca2caa9dcac9ea473933e/n3fit/runcards/examples/Basic_runcard.yml#L26

I'm perfectly ok with having both as top-level variables tbh as it is how they are consumed by validphys (so we remove the intermediate parsing in n3fit_exec