Accommodate time correlation with bunch crossings

**Is your feature request related to a problem? Please describe.**
In the far backward region, Bremsstrahlung is a big issue and it's tightly correlated with bunch crossings. Currently, background is embedded with a uniform distribution.

**Describe the solution you'd like**
Place background events at realistic timing, with realistic smearing.

**Additional context**
Simon already gave a good writeup of the solution in his contribution here.
https://indico.bnl.gov/event/20343/
This just needs to be converted to code, and the correct parameters used.