Update G4DarkBreM with configurable A' scaling and decay

[RobMina]

To be merged after LDMX-Software/G4DarkBreM#33 (which contains the updates to the G4DarkBreM submodule).

[tomeichlersmith]

Thank you for linking the PR into G4DarkBreM, I am going to mark this as a draft until we can have this PR be updating G4DarkBreM to a new release version.

(It is still helpful to have this PR open however since then we can still make sure G4DarkBreM's updates are compatible with ldmx-sw.)

[tomeichlersmith]

Alright :) with the G4DarkBreM updates merged and a new version released, let's update the submodule reference here to be the tagged release version and then we can re-run the tests and do a final review prior to merge.

The only test that failed when you first opened the PR was the C++ formatting test, so I've included the instructions on how to format your C++ code as well.

Set G4DarkBreM to v2.1.0

git -C SimCore/G4DarkBreM fetch --tags
git -C SimCore/G4DarkBreM checkout v2.1.0
git add SimCore/G4DarkBreM
git commit -m "put G4DarkBreM on v2.1.0"

C++ Format

You only changed one C++ file and if you were to look at the logs of the format check you could probably figure out what to change in your source file to align with the C++ Google style we use, but there is a tool in the container that does this for you.

ldmx clang-format -i SimCore/src/SimCore/APrimePhysics.cxx
git add SimCore/src/SimCore/APrimePhysics.cxx
git commit -m "format my C++ edits in Google style"

[tomeichlersmith]

All the checks passed and things look good from my point of view!

@horohoo I requested your review as well to double check that this includes the updates necessary to do the studies you would like to do. I'm happy to merge, just want to make sure we didn't forget anything obvious.

@RobMina After you get a second approval, you can click the "Rebase & Merge" button and then your updates will be included in trunk!

[horohoo]

Attached are a few validation plots using the ldmx-sw SampleValidation DQM class. A simulation of 10k events with a 5 MeV A' mass was used. Dark photons are being produced via dark bremsstrahlung and they are decaying to e+e- pairs, so the physics we want to simulate works well. Things look good to me!

[RobMina]

I don't have write access to this repo, so someone else will need to push the merge button to complete this PR.