Adding PDG ID remapping function for MGC

Zach Marshall requested to merge zmarshal/athena:21.6_MGC_PDGID_Remap into 21.6

This adds functionality for PDG ID remapping in a generated LHE file. There are a number of places where the PDG ID has to be modified, and we have to be somewhat careful with both IDs and anti-IDs. This should handle both.

Beta feature for the time being.

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