Implement replica filters for monte carlo in vp

Created by: scarrazza

Issue by Zaharid Wednesday Oct 04, 2017 at 09:51 GMT Originally opened as https://github.com/NNPDF/validphys2/issues/56

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This should be a rather trivial translation of my notebook, since it used the validphys API anyway.

It is important to have it integrated here so it is nice and reproducible.