Move simulation logic to an external python script

This MR migrates the body of the simulation logic into an external python script. For this MR I tried to minimize behavioral changes and cleanup. There's a lot more that could be done in terms of using native python instead of shelling out for everything, moving things into subroutines etc., but I tried to keep this to the bare minimum to set us up so that future logic can be directly written in python (without having to try to encode it in yml and shell or calling out to over-specialized helper scripts with awkward interfaces)

I did end up rewriting the locale splitting logic in Python, because I had trouble getting the shell escaping right, and that piece is particularly better handled in Python.