Allow dynesty to run with multiprocessing
This MR will allow users to pass a
dynesty and this will then be used as the number of processes in a
This implementation is very much inspired by !750 (merged).
It required adding a couple of helper functions to avoid attempting to send unpicklable methods.
I found a roughly two-fold increase using three threads on my (slightly overworked) local machine. It'll be good to explore how this scales.