add stacking scheme for bulk electron tracks

[ManuelHu]

for now, this is just meant as a reference for (manual) testing

[tdixon97]

Wow, nice work Manuel! I think this will have a huge impact on many simulations... not just optics but eg. radiogenic sims of far sources (cryostat?)

[ManuelHu]

the following macro is helful for testing:

/RMG/Output/ActivateOutputScheme VolumeStacker

/run/initialize

/RMG/Output/Germanium/EdepCutLow 10 keV
# note: this does now _not_ just affect the photons, but also the others
/RMG/Output/Germanium/DiscardPhotonsIfNoGermaniumEdep true

/RMG/Output/VolumeStacker/VolumeSafety 0.1 cm
/RMG/Output/VolumeStacker/VolumeName lar

however, the code does not yet work. Apparently, ComputeSafety always return 0...

[EricMEsch]

I just wanted to ask if you can not just use RMGOutputTools::distance_to_surface(pv, position) instead, but seems like you already adjusted that.

[tdixon97]

I did some tests and did not observe a significant speed-up.
I wonder if this can related to the "stages" in the stacking action (this I never understood), in other words.

If we do not stack some e- tracks, for example they are within safety what happens to the optical tracks produced by the e-s, does the usual optical stacking apply?
I am wondering since I found if I set safety quite high I actually get a slow-down of the simulation (factor of 2)

[ManuelHu]

If I did not introduce any error in the logic, we should still have a two-stage stack in the end (there is actually no way to stack into higher stages in a sane modular way... This really is a state machine in the end.)

1. stage = 0 -> everything immediately to track
2. stage = 1 -> everything not immediately to track (photons until now, also now e-/e+ in bulk)

The important option is /RMG/Output/Germanium/DiscardPhotonsIfNoGermaniumEdep true, because that generally controls whether stage 1 will be transitioned to, depending on the germanium energy.