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.

n3fit/src/n3fit/msr.py

@@ -152,10 +152,14 @@ def compute_arclength(n3pdf):
 def compute_integrability_number(n3pdf):
     """
     Given the layer with the fit basis computes the integrability number
-    for the distributions V, V3, V8, T3, T8
+    for the distributions V, V3, V8, T3, T8 defined in vp2
     Parameters
     ----------
         n3pdf: pdf function has received by the writer or ``pdf_model``
+
+    Returns
+    -------
+        a list of floats
     """
     ret = integrability_number(n3pdf, [1.65], flavours=['V', 'T3', 'V3', 'T8', 'V8'])
     return ret