Proper multi-threading

[aimalz]

My tests currently run in parallel via external scripts, not using an inherent chippr mode. The reason is that the function that evaluates the posterior over n(z) is not pickleable, so emcee multi-threading doesn't work. This issue is for either choosing a sampler that somehow doesn't care about this shortcoming (I suspect there may not be such a thing) or making the function pickleable. It's a low priority for the scopes of paper 0 (the model and proof of concept) and paper 1 (demonstration on toy test cases) but essential for paper 2 (more realistic and/or real data at scale).

[aimalz]

Note: I will check if I can use dill/pathos/pyina to resolve this issue.