RSG to GamGam Fragment for RelVal

Added the gen fragment for the RSG to Diphotons for RelVal Production. Exotica needs high pt diphoton samples for online/offline dqm.

RSGravitonToGammaGamma_kMpl01_M_3000_TuneCUETP8M1_13TeV_pythia8_cfi.py

uses pythia8, with the CUETP8M1 tune