Functionality for storing LHE event record in derivations

This adds several pieces of functionality necessary to store LHE event records in derivations:

• We add back a switch for Pythia8 running to include the LHE record as a HEPMC3 attribute (noting that this only works in HEPMC3, so we should warn users if they try this with HEPMC2)
• During xAOD Truth conversion, we create an additional truth particles collection in the case that the attribute is found. Tested and confirmed that if no attribute is found, no collection is created (not even an empty one). Again, only works in HEPMC3 mode.
• In derivation making, cleaned up the LHE record pieces; since the collection is made during xAOD truth conversion, all we have to do is write it out, we don't need any further tools.

Changes the truth derivation output for specific cases of event generation.

Added a switch for AOD content, so that if someone adds

genSeq.Pythia8.SaveLHERecord = True
evgenConfig.specialConfig = 'HasLHERecord=True'

to their evgen job options this will be created and also be passed into the AOD.