Send all steps back to Geant4

[sethrj]

To improve Geant4 integration at the cost of more memory bandwidth we could add an option to send all steps back to Geant4 as the AdePT team have proven feasible (see apt-sim/adept#356 ). This would obviate #1594 .

• [ ] Buffer all steps instead of just hits
• [ ] Call user stepping action

Additionally we might need/want to call the "stacking" action to indicate a new secondary.