Basic loop demo integration

[sethrj]

Propagate through a VecGeom geometry with multiple materials. Include at least one each of continuous and discrete processes. Compare simulation results and timing against Geant4.

Requirements:

• [x] Physics process management (#28)
• [x] Linear propagator interface (#66)
• [x] Material<->volume mapping (#74)
• [x] (Possibly) initialization (#52)

Add the following kernels:

• [X] Pre-step kernel (sample and set remaining mfp if unset, call calc_tabulated_physics_step)
• [X] Along-step kernel (determine actual step distance with geometry vs physics; propagate; call calc_energy_loss)
• [X] Post-step kernel (decrement distance-to-interaction, call select_model)
• [x] Interactor launches. @sethrj
  • Get direction from geometry states
  • Template ModelInterface on MemSpace and overload model interact method to work with host pointers
  • Loop over all model IDs and fire off kernels
• [X] Postprocessing secondaries and interaction results
• [x] Initializing tracks from primaries/secondaries @amandalund

Missing pieces:

• [x] Geometry and physics input files
• [x] Source definition (use HepMC3 input) @stognini
• [x] Missing Process classes (Rayleigh?, bremsstrahlung)

[whokion]

For a minimal set of EM processes for this multi-processes demo, Compton-Bremsstahlung-Ionization may be a good choice for simulating a self-contained (electron-gamma) cascade, which contains both discrete processes (all of them) and continuous energy loss processes (brem and ioni) with input of either photons or electrons: 1) starting from photons, it will undergo Compton (scattering and electron production) -> electron (energy loss by ionization, delta-ray by ioni and photon production by brem) -> repeat this cycle until both (primary and secondary) photons and secondary electrons reach to energy thresholds of processes, 2) starting with electrons, a similar cycle, but competing (brem+ioni) first followed by Compton and so on. It is a good demo with a complete simulation cycle for demonstrating performance measurements and basic validations if the total number of steps and tracks are statistically comparable to results from an equivalent Geant4 application.

[sethrj]

Rescoping this and calling it done, even though there's still clearly work to be had in the demo loop.