Custom tracking manager backport
This merge request backports the custom tracking manager functionality implemented in G4.11. This is needed for the integration with AdePT as well as for G4HepEm integration (using G4HepEm tracking manager).
This merge request backports the custom tracking manager functionality implemented in G4.11. This is needed for the integration with AdePT as well as for G4HepEm integration (using G4HepEm tracking manager).