Switch muon scale factors to correctionlib

We've been using muon scale factors taken from ROOT files. However, from Run 3 onwards they are only provided as correctionlib JSON. We have to switch if we want to support Run 3.

This commit duplicates the GenericSFApplier to add a version based on correctionlib. The idea is to migrate the other users (photons) to correctionlib and then remove the old class.

I've tested that we get identical central values for a small subset of 2018 MC.

A known drawback is that muons at eta=-1 and +1 will be considered uncorrelated for the statistical uncertainty even if the scale factor was determined with the two bins combined. This can only be solved properly by adding support for binning information in correctionlib.

See #136