Replace multiprocessing with pathos

[agartland]

Currently we use python's built-in multiprocessing to spread computations across multiple CPUs. This requires that the metric function be globally importable, which is a limitation of pickle, which is used by multiprocessing. The package pathos offers a essentially drop-in replacement for multiprocessing which uses dill to serialize the objects that are passed to worker processes, which would allow for locally defined and more complex functions to be used as metrics. Currently this is not a major limitation, but if this issue crops up again in the future I think this would be an easy update to make.

Links with info:
https://github.com/uqfoundation/pathos
https://medium.com/@emlynoregan/serialising-all-the-functions-in-python-cd880a63b591