Evaluate only unique xgrids

Created by: scarlehoff

Deals with #1586 (closed) in a very generic way so that one is still allow to mix situations in which not all xgrids are equal but some of them are.

For now only done for the fit. Once tests have run and it works there, I'll try to add some kind of cache to the vp PDF so that we can avoid calling lhapdf once per dataset (which I cannot promise because it is both trivial to do and trivial to do it in a disastrous way).