Make PassBackG4 output match in Athena and AthenaMT
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