Draft: Initial version of Geant4SimulationAlg to run G4 in a parallel thread to Athena/Gaudi

This MR adds a new algorithm and infrastructure to run G4 in a parallel thread to Athena/Gaudi. G4 then creates it's own worker threads. Mutual bloacking of the respective Athena and G4 threads allows the event simulation to progress with only one of the two threads running at the same time. For now, The G4 B4c example is used instead of the actualy ATLAS detector