Expose positivity threshold in the runcard

Created by: scarlehoff

This is the maximum value that the replica's positivity loss can have to get a POS_PASS tag in .fitinfo. By default it has the conservative 1e-6 value, but now it can be changed directly from the runcard as sometimes might not a good idea to be too strict. A comparison below:

I used as baseline the 130920-nnpdf40_jcm_iterated fit from Cameron, for which 66% of the replica passed. This was with a threshold of 1e-6.

I've launched a fit with 1e-4 and another one with 1e-3, the results are below:

For 1e-4, 74% of the replicas passed positivity

https://vp.nnpdf.science/VLGnw8h8SPOVI4HxJ0YaXA==/

For 1e-3, 90% of the replicas passed positivity.

https://vp.nnpdf.science/A1WNzi7oQNewiGzRDF3isA==/