Draft: Investigating splitting the concept of HepMC::barcode into unique id and origin

John Derek Chapman requested to merge jchapman/athena:Barcode2ID_master into master

Will go through the code and slowly label the places where barcode is used as a unique id (truth matching) and where it is checked to indicate the origin of a particle (finding lots of bugs in the latter).

Tagging @zmarshal and @abuckley

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