Matched cuts

Created by: Zaharid

This computes the matched cut plots:

https://vp.nnpdf.science/NlltmlyWRRqCtSeJbi1xIQ==/

It would be good to understand if the total shift can be represented in some reasonable way that is of any use.

For example one could take the ratio to the thory covmat. Also the plot above would have to be recomputed with different PDFs (not that it matters since you can't see anything in it at the moment).

This of course depends on #284.