visible decay and A' kinematics scaling

[RobMina]

Changes to the processing of A' from dark brem vertices: scaling momentum based on MadGraph vertices and allowing decay to e+/e- within the detector simulation. A' momentum scaling uses the same configuration options as for recoil electrons, but is toggled off by default. A' decay is configured using a new configuration setting specifying either no decays (default), physical decay using configurable lifetime, or flat (in lab frame) decay distribution with configurable minimum and maximum flight distance.

[tomeichlersmith]

I am looking into this now :) It compiled within the ldmx-sw development environment out of the box which is a great sign.

A new PR naturally reveals some issues that I need to patch before I can successfully test this.

• I need to update the example libraries to include the target Z which is required as of G4DarkBreM v2.0. Currently generating right now, will hopefully be done soon(ish).
• Since this introduces a new feature, I'll want to resolve update doc publishing to use actions rather than from-branch #29 before (or shortly after) this PR so that the online docs include the features you're introducing.

To help things move along, can you share how you validated your developments? What plots did you make to confirm to yourself that this was working as intended? (Sidenote: if you haven't validated this personally yet, we may want to mark this "as draft" to reflect that things might change as you validate the developments. Plus, I'll get a notification when you mark it "ready for review" so you won't have to poke me again when you want a review.)

[tomeichlersmith]

As a first-pass, no-testing review, things look good.

My biggest request is documentation related. Please add these new configuration parameters to the summary table in the documentation.

G4DarkBreM/docs/configuration.md

Lines 28 to 30 in e272adb

	Parameter | Required | Location | Description
	---------------------|----------|-----------------------|-------------
	A' Mass | Yes | G4APrime | Mass of dark photon in MeV

The inline code documentation appears to be good and up-to-date ✔️

[RobMina]

Validation was done using scripts in this repo that includes a fork of ldmx-sw as a submodule. Specifically, this jupyter notebook was used to validate the A' scaling procedure and this one was used to validate the decay handling (including the most recent updates). The scaling plots are all available in this google drive.

[tomeichlersmith]

These look amazing ✨ Thank you for all of the detail and sharing all of the plots! These are exactly the plots I was going to make and so that saves me a boatload of time.

[tomeichlersmith]

Looks good 👍 I just want to add more detail to some of the documentation, but I'm happy to merge this and let the ldmx-sw validations check to make sure it doesn't change our current signal production mechanism.