Initial setup for G4 FastSim hooks - #15698

This commit provides the fast-simulation hook that O2 was missing, in a new
Detectors/FastSim module, plus one toy model that exercises it end to end. The
feature does nothing unless G4.fastSimModels names a model.

- o2::fastsim::G4RunConfiguration overrides CreateUserFastSimulation, which is
  the only piece geant4_vmc needed and O2 did not supply.
- TG4FastSimulationPhysics is already registered unconditionally by
  TG4SpecialPhysicsList, so the specialProcess string is unchanged.
- FastSimModel::DoIt is shared plumbing: it measures the distance to the
  envelope surface, kills the incident particle, stacks what comes back and
  books the energy difference as a deposit.
- A model implements sample(), which maps the particle that entered to the
  particles that leave.
- ToyAbsorberFastSim returns one particle carrying on in the incident direction
  with an exponentially attenuated energy.
- Regions are named as tracking media in G4.fastSimRegions.
- With G4.fastSimModels empty the run configuration returns nullptr and the
  behaviour is identical to before.

Usage:

  o2-sim -n 10 -g pythia8pp -e TGeant4 -m PIPE ABSO \
    --configKeyValues "G4.fastSimModels=toyAbsorber;G4.fastSimRegions=ABSO_AIR_ENVELOPE"

The steps inside the region disappear from the step log, which is the saving:
a fast step defaults to AvoidHitInvocation, so Geant4 does not call the
sensitive detector and TVirtualMCApplication::Stepping() is not invoked. That
is correct for a passive envelope, which has no hits to lose.

This adds run/SimExamples/FastSim_Absorber, which runs the same five events
through PIPE and ABSO twice, once with detailed transport and once with the
toy model, and compares the number of tracks.

- run.sh performs both simulations and the comparison.
- countTracks.macro reports tracks per event of an o2-sim output.
- README.md says how the feature is switched on, what the toy model does and
  why the region is named after a tracking medium.

This adds the numbers from a first run of the example, and says what they do
and do not show.

- Five pp events with PIPE and ABSO: 3544 tracks per event with detailed
  transport, 3160 with the toy model.
- Transport time is 29.8 s against 28.0 s, which is not a performance claim:
  only the forward cone of a minimum-bias pp event enters the absorber, so most
  of the transport in this setup happens outside the region.
- The geant4_vmc line confirming that the tracking medium resolved to its
  material is quoted, because a wrong medium name fails silently.

This documents a limitation found by running the example, and withdraws the
reading of its track counts.

- ABSO_AIR_ENVELOPE selects a region containing AFaM and nothing else: the VMC
  special cuts make every logical volume a root of its own material's region,
  and Geant4 stops propagating a region at any such daughter.
- AFaM's daughters touch its surface, so a track entering the absorber lands in
  a daughter and never has AFaM as its volume.
- A 20 GeV muon with /tracking/verbose 1 steps through the absorber identically
  with and without the fast simulation, showing only muIoni, eIoni,
  Transportation and specialCutForElectron.
- The two runs' random sequences diverge before the absorber, so the track
  counts previously recorded do not measure the model.