Manual sweep of athenaprivate1!16317 from 21.0 to master: workaround for the issues seen when passing zero lifetime particles to Geant4 (ATLASSIM-3313)

The ZeroLifetimePositioner works around the case where a neutral particle oscillates into its anti-particle in one GenVertex then immediately decays in a second GenVertex at the same position and time. This is currently a problem seen when B0/B0bar (511/-511) particles with pre-defined oscillations and decays are fed into Geant4, which cannot currently handle the concept of a zero-lifetime particle. As Geant4 does not implement any processes for these particles other than the pre-defined decays currently and the particles are neutral, it is safe to shift the point at which the particle oscillates into its anti-particle back along the (straight-line) trajectory a bit. For simplicity halfway between the production point and the oscillation/decay point has been used. The decay position is left unchanged.

With this change Geant4 then successfully simulates the decay in the required position.

In QS sim jobs only, after the TruthEvent is created from BeamTruthEvent, but before it is passed to the InputConverter, ZeroLifetimePositioner is used to apply the work-around for zero lifetime B0/B0bar particles. After simulation is complete ZeroLifetimePositioner is used to remove the work-around by moving the oscillation vertex back to the decay vertex position.