adding xAOD truth to HepMC GenEvent conversion
The purpose of this change set is to pull some logic (e.g. the
TruthConverter) from 21.2 and use (+ extend) it in
Rivet_i to allow AODs and TRUTH1 DAOD to be read in and their xAOD::Truth format to be converted back to HepMC::GenEvent, so that it can be passed on to Rivet.