Allow dynesty to run with multiprocessing

This MR will allow users to pass a queue_size to dynesty and this will then be used as the number of processes in a multiprocessing.Pool object.

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.