Add `resample_negative_pseudodata` to the `sampling` namespace

[scarlehoff]

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

nnpdf/n3fit/runcards/examples/Basic_runcard.yml

Line 26 in e9f2763

resample_negative_pseudodata: True

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

[andreab1997]

Ok @scarlehoff sorry for the delay, I will try to take care of this asap

[RoyStegeman]

Should we remove the separate_multiplicative key?