Allow kaon physics to be configurable at runtime by the user

[EinarElen]

This PR adds the ability to change the kaon physics at runtime by modifying the properties of the G4KaonMinus and G4KaonPlus particle definitions rather than doing it manually in the Geant4 sources. This should significantly simplify the study of challenging kaon backgrounds, especially together with techniques like https://ldmx-software.github.io/Alternative-Photo-Nuclear-Models.html

I also add "Decay" as one of the processes registered in the ProcessMap so that you can tell that a daughter particle came from a decay.

To verify that this works, I used a configuration like the following and some checks in the SimObjects DQM code to check basic things like "if I force only one decay type, I always get daughter particles of that type" etc

from LDMX.Framework import ldmxcfg
p = ldmxcfg.Process('test')

p.maxTriesPerEvent = 1

from LDMX.SimCore import generators as gen
from LDMX.SimCore import simulator
from LDMX.SimCore import kaon_physics
mySim = simulator.simulator('kaon_test')

gun = gen.gun('kaon_gun')
gun.particle='kaon+'
gun.position = [0.,0.,0.]
gun.direction = [0.,0.,1.]
gun.energy = 0.
mySim.setDetector('ldmx-det-v14-8gev')
mySim.beamSpotSmear = [20.,80.,0.]
mySim.description = 'ECal PN Test Simulation'
mySim.generators = [gun]
mySim.kaon_parameters = kaon_physics.KaonPhysics.upKaons()
mySim.kaon_parameters.kplus_lifetime_factor = 1/10000000000000000.
mySim.kaon_parameters.kminus_lifetime_factor = 1/10000000000000000.
mySim.kaon_parameters.kplus_branching_ratios = [0., 1., 0., 0., 0., 0.]
p.sequence = [ mySim ]

##################################################################
# Below should be the same for all sim scenarios

import os
import sys

if 'LDMX_NUM_EVENTS' in os.environ:
    p.maxEvents = int(os.environ['LDMX_NUM_EVENTS'])
else:
    p.maxEvents = int(sys.argv[1])
if 'LDMX_RUN_NUMBER' in os.environ:
    p.run = int(os.environ['LDMX_RUN_NUMBER'])
else:
    p.run = 1

output_base = 'kaon_test'
if len(sys.argv) > 2:
    output_base = sys.argv[2]


p.histogramFile = f'{output_base}_hist.root'
p.outputFiles = [f'{output_base}_events.root']

import LDMX.Ecal.EcalGeometry
import LDMX.Hcal.HcalGeometry


from LDMX.DQM import dqm

p.sequence.extend([
    dqm.SimObjects()
])

This resolves #106 and part of #107

[bryngemark]

Overall looks like a great, clear implementation of changing kaon properties. I just have the one neutral kaons comment.

[bryngemark]

Right here, or in a separate class? Would it make sense to add neutrals while we're at it? If not, should we call this ChargedKaonPhysics? I'm trying to make sure future me is not confused by this and I can imagine several ways. My preference would be to deal with all kaons here if it is easy enough, and just toggle which kaons one would like to modify with a configuration parameter.

[EinarElen]

Right, that's a good point. It won't be as "clean" with the neutral kaons since they don't have symmetric decay channels like the charged ones do but it is straight forward to do

[EinarElen]

Should be there now!

[bryngemark]

Sweet, thanks!

[tomeichlersmith]

This is so cool! Thank you for looking into this so quickly :) I am very excited that we can move this specialization into runtime from compile-time.

Most of my comments are just thoughts and not really necessary changes.

[EinarElen]

Ok after some minor bugs in the updated python config, I've tested and it handles all the edge cases well (e.g. BR sum > 1, BR sum << 1, negative lifetimes and whatnot) and the upKaons call works!