TrkValTools: migration from barcodes to GenParticle::id()
This is part of the migration of Athena away from using GenParticle barcodes (removed in HepMC3) as a unique identifier for truth particles to GenParticle::id() instead.
Ntuple branch names have not been changed (just commented as a reminder to change them in the future), but all variables previously assigned barcode values are now assigned GenParticle::id()
instead.