Replace barcode-related magic numbers with constants
Use HepMC::UNDEFINED_ID rather than 0 for undefined barcodes.
Use HepMC::INVALID_PARTICLE_ID rather than -1 for invalid barcodes.
More preparatory changes for !67195.
Use HepMC::UNDEFINED_ID rather than 0 for undefined barcodes.
Use HepMC::INVALID_PARTICLE_ID rather than -1 for invalid barcodes.
More preparatory changes for !67195.