Make PassBackG4 output match in Athena and AthenaMT

John Derek Chapmanrequested to merge
jchapman/athena:PassBackG4MT_master into master

We already have FullG4 simulation running with SimKernelMT in AthenaMT. This gives identical to SimKernelMT running in Athena and the old SimKernel running in Athena. These setups use one G4Event per Athena event, as is done in AtlasG4.

Other ISF configurations use multiple G4Events per Athena event, however. The simplest of these is PassBackG4, where particles are passed back from G4 to ISF at the ID-Calo and Calo-Muon boundaries.

After the changes in !38056 (merged) and this merge request, the output of PassBackG4 running with SimKernelMT in Athena and AthenaMT matches the output SimKernel running in Athena.

NB These changes will alter the output of existing ISF configurations using SimKernel+ParticleBrokerSvc due to the fact that they pass all remaining particles (up to a configurable maximum) that should be processed by a given simulator, rather than just the first continuous block of particles destined for a simulator.

FYI @mduehrss, @mbandier, @mmuskinj, @zmarshal, @bwynne Should fix ATLASSIM-4763

Edited by John Derek Chapman

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