Integrate correctionlib
Related to #17 and #22 (closed) , part of #85
correctionlib loads scalefactors etc. from the CMS-standard JSON format. With cms-nanoAOD/correctionlib#60 (included in v2.0.0rc4) everything should be there to pick it up, but it needs to be tested (I'll try and open a PR as soon as I have something working).
This will probably require some experimentation (and possibly changes to propose for the correctionlib evaluator), so it's also a good opportunity to think about the get_scalefactor
interface, and how we include scalefactors in general (doc).
Edited by Pieter David