This merge request includes an initial version of McAuxEventCollection - a class to hold the extra information in McEventCollection_p6 compared to McEventCollection_p5. It also includes a dummy TP converter setup (where the persistent classes are basically identical to the transient classes).
In order to test further I really need an EVNT file or two containing McEventCollection_p6 (one of them with event numbers above the 32-bit limit). Then I can write the Algorithm to create a transient McAuxEventCollection from a transient McEventCollection and the Algorithm to merge McAuxEventCollection and a sparse transient McEventCollection generated from McEventCollection_p5 into a new transient McEventCollection object with the combined information. This can also be used to optimize the formats particularly the persistent version.
Added McAuxEventCollBuilder Algorithm to create McAuxEventCollection objects. Added PatchMcEventCollection Algorithm to restore information from McAuxEventCollection back into McEventCollection. (Both intended to be run in HepMC3-based releases. To be tested by running a step in a HepMC2-based release in between.)
Part of the work for ATLINFR-4725.
Tagging @ewelina, @mgignac, @dhirsch, @nstyles, @jcatmore, @elmsheus, @tlari, @mbandier