Adding BunchCrossingTime and PileUpType Attributes to GenEvents during pile-up digitization

1. Add "BunchCrossingTime" and "PileUpType" Attributes to GenEvent objects during pile-up digitization / presampling (HepMC3 only). These provide an alternative to the current McEventCollection structure-based approach of GenEvent event classification used when pile-up truth is written out.
2. Update client code to propagate/make use of these properties.
3. Update SimpleMergeMcEventCollTool to be able to optionally override the event numbers in the same way as MergeMcEventCollTool.
4. Switch HepMC3-based builds to use SimpleMergeMcEventCollTool instead of MergeMcEventCollTool. This drops the complicated McEventCollection structure, but should be transparent to reconstruction output.

Relates to ATLSWUPGR-167.