Update G4DarkBreM with configurable A' scaling and decay

[RobMina]

To be merged after https://github.com/LDMX-Software/G4DarkBreM/pull/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"

[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.