ZGamma corrections
Add a data-driven ZGamma correction factor estimated from a 2l+gamma control region, along with an associated uncertainty.
The prescription proposed by Sicheng (use the DY subtraction method as the main one, and add the difference between the 2 methods as a contribution to the uncertainty) is used by default, but it can be changed easily in the function.
This MR builds on top of !137 (merged) which should be merged first.