integrability check in postfit

Created by: tgiani

As agreed I ve open a new branch from the current master and I ve cherry picked the relevant commits from the other PR #1004 , which now I m going to close. I ve run a test on this branch using using the new setup, and I ve run postfit with a very high intergrablity threshold, to keep all the replicas. Here the comparison with the current best candidate https://vp.nnpdf.science/fdcPxZOdQ72VqqYNSmvZnA==/

Looks alright, the only thing which confuses me a little bit is that some distributions, like T8, are a bit different at small x, even if the runcard is exactly the same (and the postfit check is not doing anything here because I m using a very high value for the threshold). Is this expected?

@scarlehoff just to be sure, is the theory 53 you are using to produce the fits up to date?

Also, I noticed that the small-x preprocessing range for T8 is set to [0.7167,1.102]. I think that, as discussed few month ago, we should set it to [0.7167,1.00], otherwise it s not compatible with integrability.