Unweighting implementation

[toonhasenack]

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Calculation of the unweighted weights in the vp framework. Module can be called with vp-unweight "path to chi2".csv "number of datapoints"

[RoyStegeman]

Why is this a cli command and not just a vp action?

[toonhasenack]

@RoyStegeman, I followed the suggestion of @scarlehoff to do it this way.

[Radonirinaunimi]

@toonhasenack Just to echo what has been above about the re/un-weighting being self-consistent. Everything except the generation of the theory predictions should be here, ie from the computations of the $\chi^2$ to the final dumping of the PDFs. Ideally, the theory predictions should also be stored in ../NNPDF/theories.

Also agreed on this being vp actions instead of scripts.

If you want, I could take over this PR from this point.