Change LHAPDFSet to a python interface

Created by: scarlehoff

Right now we use in many places in validphys some LHAPDFSet class. As far as I can see this can be substituted for a python interface (which lhapdf provides, so in practice what we are exchanging is the layer of libnnpdf that is calling the Cpp bindings of lhapdf).

Looks to me like a low-hanging fruit for the destroyingc++ target.

Is there anything important I'm missing that would complicate this enormously? I can work on this myself since I'm playing with the integration of nnpdf and pineappl and this is one of the sticking points*.

*not directly useful for the fit, but it makes the integration between validphys and pineappl more complicated where it shouldn't be