Feature particle gun

[TiagoTAlves]

This is an initial attempt at what we are calling a "Particle Gun".

A particle-gun.mac geant macro file creates particles inside a volume encompassing (only) ND LAr in the geometry/nd_hall_with_lar_tms_sand_TDR_Production_geometry_v_1.0.3.gdml file. The macro also constrains the direction of the travelling muons, (Pz>0), whilst also allowing full freedom in Px and Py. (This was a specific design choice with no basis on physics and can be changed if needed).

This macro file is only a template however and will not run directly. For ease of use, a user will be able to use environment variables to change the energy of these particles, using $ARCUBE_ENERGY_MINIMUM $ARCUBE_ENERGY_MAXIMUM, as well as the flavour of particle, using $ARCUBE_PARTICLE_TYPE. The energy distribution for the moment is flat, but manual changes can be made to this geant macro file if a different distribution is required.

Changes to the run_edep_sim.sh were also required but an attempt was made to make sure no current workflow would be affected by these changes.

Only if the environment variable $ARCUBE_BEAM_TYPE = "particle_gun", will any new environemnt variables would need to be used to continue with production.

The nEvents, under a normal production, would use the events from a Genie file, but if you need a Particle Gun you would be able to make these changes yourself, using the ARCUBE_EXPOSURE environment variable (same env var used in other bash scripts).

Finally a test, script to run a particle gun edep-sim production was added to test it.