Draft: Fixes for Pile-up SDO creation during MC Overlay jobs

In ATLASSIM-7247 it was reported that on 10th July 2024 SDO creation began to fail in MC Overlay jobs run in the main nightlies - it works in 24.0 though. I suspect that this is related to some aspect of the barcode migration, but cannot find the exact cause. SDO creation still works in pile-up digitization, so there must be another cause.

Initially it was noted that CopyMcEventCollection produced an McEventCollection with different event numbers than SimpleMergeMcEventCollTool. This has been fixed and signal GenEvents now have event_number = 0. Background GenEvents have been left as they were for now - need to find a job with pile-up truth information to test this further and update if necessary.

Next it was noted that the Bkg_TruthEvent McEventCollection was being picked by HepMcParticleLink as the McEventCollection to which all links should point. In the case of MC Overlay jobs (without pile-up truth) then the HepMcParticleLinks in the input SimHits should point at Sig_TruthEvent, while the SDOs should point at TruthEvent. First Sig_TruthEvent was added to the list of possible keys. This had the effect of making all links point at Sig_TruthEvent (even the redirected ones). Tweaking the static HepMcParticleLink::getRedirectedLink method to re-roll the SG::DataProxy finding allowed the redirected links for hard-scatter GenParticles to point at TruthEvent.

At this point I think that the updates in this MR improve the output sufficiently to be worth merging, but further follow-up may be required in the future.