Factorization scale variations for grids.

[scarlehoff]

At the moment we have only implemented renormalization scale variations for grids that are missing the scale dependence.
It would be good to add the factorization scale dependence as well.

This might be unnecessary if we get the last few grids with scale variations that we are missing but I'm opening the issue just to keep this issue in mind.

We should have already the information necessary for NLO.

(I'm assigning myself to it since I don't want to inflict this pain onto others, but cc possible interested parties @cschwan @felixhekhorn @andreab1997)

[alecandido]

The optimal way of doing it, it's to take the Mellin-inverted splitting functions from EKO. But the API for this is missing in EKO itself (even though you can also implement outside EKO, paying the price of using internals).