MagicNumbers.h: Adding new HepMC::UNDEFINED_ID constant (ATLASSIM-6906)
Adding new HepMC::UNDEFINED_ID
constant to TruthUtils/MagicNumbers.h
replace Barcode::fUndefinedBarcode
from BarcodeEvent/Barcode.h
.
Will migrate clients in a separate merge request.
Part of the work for ATLASSIM-6906.