Further clean-up/improvements to TruthUtils headers
- Use status and barcode helper functions in
HepMCHelpers.h. - Rename
SINGLE_PARTICLEconstant toSINGLE_PARTICLE_BARCODEfor clarity(update clients correspondingly).
HepMCHelpers.h.SINGLE_PARTICLE constant to SINGLE_PARTICLE_BARCODE for clarity(update clients correspondingly).