Generators: reinstate xAOD::Truth-to-HepMC::GenEvent conversion
This change set is to port the on-the-fly conversion of xAOD::Truth to HepMC::GenEvent, which allows users to run Rivet over AODs (as well as derivations that keep the full truth record), from 21.6 to 22.6/master. The master-branch version of the truth-format conversion tool was also still missing support for multi-weights.