separate n3fit package

Created by: wilsonmr

I'm pretty sure this is broken still. I think it might actually be better to make two subpackages: validphys and n3fit and then a metapackage nnpdf which depends on them. It's not clear to me whether subpackages can depend on each other, but I will try testing that next week.

I think if I went down the metapackage route it would mean I don't have to c+p # [linux] on every line for the n3fit package.