Remove libNNPDF::LHAPDFSet for a vp-based class

Created by: scarlehoff

Turns out it wasn't such a low-hanging fruit, but closes #1420 (closed)

Some caveats (the reasons why this is still a draft):

• There are a few very fundamental places where libNNPDF needs to still be used because there are libNNPDF objects asking for a PDF. I need to check whether I can find a workaround for some of them, at present I'm happy with my solution for those (as they also moved from libNNPDF to python it will be as easy as removing the as_libNNPDF() call.
• grid_values in python is not as slow as I initially thought it will be. I just tried with the most naive loop-nesting and it was actually reasonable, but I need to actually benchmark it. If it happens to be really slow for most use cases I have a few solutions in mind that will work for us, but maybe it is not even necessary.

RE the current failure in the tests: is due to using float64 thought, when using vp-nextfitruncard there is a difference in the 4th digit. Reasonable. Maybe more failures will occur as I move forward though.

TODO:

• Add an interface for alpha_s and xfxQ such that it can be easily interfaced with the python interface of pineappl

Despite being marked for review, it cannot be merged until the example resources are updated.