G4DarkBreM update and EaT-related Patches

[tomeichlersmith]

This PR is a few disparate patches related to my work studying the signal in the Ecal-as-Target analysis. They are relatively small changes that I have been using for several months and so I think it is fair to merge them in.

1. Put another entry in the event header parameters corresponding to the elemental-Z of the dark brem interaction. It will be -1 if the dark brem did not occur. This is very helpful for analysis of a signal sample to make sure the distribution of elements on-which the dark brem occurred makes sense.
2. Update G4DarkBreM to v2.0.0. This major release of G4DarkBreM was made to support multiple-Z dark brem reference libraries which is helpful for making the EaT signal more realistic as it is distributed across many materials with widely varying Z in elements.
3. For the v14 detector, the ecal motherboards were not named in a way that was matched by the biasing operator attachment function so I patched that function to be able to pick up these volumes. They are thin PCB so including them does not affect the distributional shapes of any of the physics variables I was looking at, it just smoothed out the weight distribution a bit.

[EinarElen]

Extremely minor comment comment aside, this looks reasonable to me. Only thing would be the default -1 value which could cause some issues if someone isn't reading the docs and tries to do something like taking the average Z of all event headers but I can't think of a better option since it has to be stored.