adding Calo/CaloTools/GammaPi0XGBoostTool.h, GammaPi0XGBoostTool.cpp, XGBClassifierPhPi0.h, XGBClassifierPhPi0.cpp, CaloHypoEstimator.h, CaloHypoEstimator.cpp files to provide gamma-pi0 separation tools
@deschamp @jmarchan FYI. I am adding you because you should review this code