BEAMS3D: Ability to mark self-generated markers with population index

[lazersos]

This added the capability to use DEX_BEAMS to put each user defined marker in a separate population. Useful for quickly generating heating benchmark plots. If DEX_BEAMS is not specified the code should behave like it always did placing all user generated markers in one population.

[kudav]

I think the way it is right now results in some unexpected behavior. I would like to run a slowing-down benchmark with 50 particles with individual pitch values and group these together to have eg individual heating profiles. Loading them in works fine, but using dex_beams is restricted to the first 32 particles:
Running with
DEX_BEAMS = 31*1 2
produces a run with 50 particles, with the first 31 in the first bin and the following particles in the second bin, for which there is no individual control

Running with
DEX_BEAMS = 31*1 2 2
Produces the following error:

----- Input Parameters -----
ERROR reading namelist BEAMS3D_INPUT from file:
input.ORBITS_slow
Invalid line in namelist: DEX_BEAMS = 31*1 2 2

Which does not help to resolve the underlying issue.

My point is, dex_beams for particles fulfills a different purpose (grouping potentially many markers in a few bins) than for the beamlet model (assigning BEAMS3D beams a beamlet index).

[lazersos]

Just change MAXBEAM=50 for your case and recompile. Restructuring the code would be required at this point and I don't want to do that. This is a simple fix to allow individuals to run an edge case.