n3fit should use existing code for arclength

Created by: Zaharid

We have

https://github.com/NNPDF/nnpdf/blob/master/validphys2/src/validphys/arclength.py

to compute arclengths (I think it is a bad idea to look at arclengths but that's another discussion). We are duplicating some of that functionality in the n3fit code. There are obvious reasons why we shouldn't do that, not least because it is a non trivial computation. In fact the one in validphys is wring and someone should look at it. See #156 (closed). Of course that doesn't mean that the one in n3fit is right. There is no reason to believe that. We should have one well tested implementation instead of two questionable ones.