Draft: Remove hard-coding of the PDG ID scheme in the Geant4 Monopoles config

Hello,

this MR is intended to remove the hard-coding of the CODE variable, which represents the PDG ID of the particle, from the load_files methods in the Geant4 Monopoles config.
In the existing version, that CODE variable can be overwritten by setting the InteractingPDGCodes flag in any other part of the code, just not in the load_files methods. This is fixed now.
To reduce code-duplication, I also put the assignment of the InteractingPDGCodes in case it is not yet filled into separate methods.

I would also have a quick question: We would like these changes to be present in r23/r24 so it can be used for the production of the Run 3 signal MCs we are currently preparing. So I was wondering if requesting to merge into main is fine or if I would best target another branch?

- Judith