Implementation of GNNC tool

This MR provides the functionality to use a NN for the global calibration stage of the jet calibrations (replacing the GSC). In addition to the NN, it applies a small pT correction from a histogram, similarly to how this is done for the p-splines in the EtaJES stage.

I have tested that this code replicates the behavior seen in my local implementation of the calibration.

Tagging @cdelitzs @mswiatlo @dportill for their information.