CMSZDIFF12 chi2

Created by: scarrazza

Following our discussing today, I found the commit which deteriorates the chi2 of CMSZDIFF12 from 1.32663 to 3.53007 when using NNPDF3.1 NNLO.

https://github.com/NNPDF/nnpdf/commit/97c4c5b3a302f8921ff5430ffd69616b1c398bdd

As you see, this derives from the fix of the cholesky decomposition, not sure exacly how this affects only the zdiff computation.