Potentially swap out `concurrent.futures` with `ray` for multiprocessing backend.

[weinbe58]

if using python's built in multiprocessing library or components built on that, e.g. concurrent you must wrap your parallel portions of code with if __name__ == "__main__": to try and protect against infinite forking of a process. we're currently going to be using this as a backend in #421.

While this isn't overly cumbersome I feel like we should explore the package ray for executing emulator tasks in parallel. Thanks to @Roger-luo for pointing this library out.