Bethe-Heitler muon overlay plumbing

[dervj]

We want to create a framework for BIB overlay where we can toggle sampling of BH muons

[madbaron]

I think BH muons could be simply added as an additional group in OverlayTiming, like do with the combined overlay of BIB and IPP in #43

To test before the Overlay PR is merged, one could use ghcr.io/muoncollidersoft/mucoll-sim-ubuntu24:post_3_1_test