Add final size calculations

[sdwfrost]

@slwu89 had already implemented the final size of a stochastic SIR using Sellke's construction. It would be nice to have final sizes for a variety of different implementations. This could also be another illustration of DiffEqCallbacks.jl. I'm not sure whether to put each in their own existing workbook (which would probably be the easiest to maintain) or to collate final sizes into a single notebook.

Papers of interest:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3506030/
https://royalsocietypublishing.org/doi/full/10.1098/rspa.2012.0436

[slwu89]

Haven't looked at what algorithms are implemented yet, but this new R package may be of interest https://epiverse-trace.github.io/finalsize/

[sdwfrost]

Thanks Sean! I'd already seen Joel's paper, but I didn't know about the R package. I just added a multigroup SIR model, and I have a working finite state projection example too.