Add regression (and some new validation variables)

Adding regression. Performance seems ok at first glance, I'll make some real plots for Monday.  Two things to note especially:

1. There are now additional steps in README.md to get the regression code, which is currently in two additional repositories.  Eventually we should plan how to merge it into flashgg or CMSSW as appropriate, but this is an ok placeholder.  

2. Code to handle energy "history" has been added to MicroAODFormats/src/Photon.cc - it is stored in userFloats for now, but the structure is set up in such a way that we can handle it better later and only modify a few functions.