Parallelized MCMC runs are broken on (at least) MacOS with python>=3.8

pyDARM parallelized MCMC runs are broken on MacOS with python>=3.8. The tests fail because of the way that the multiproccessing and emcee libraries handle global variables is incompatible with the way this is done in MacOS. Fortunately, this still works with python-3.7.

An open issue is on the emcee github page.