multiprocessing Python 3.8 macOS

[joachimwolff]

Python changed in version 3.8 for macOS the start method from 'fork' to 'spawn', because fork is considered as unsafe. However, our usage of multiprocessing leads to crashes with the new start method. A workaround is to set the start method manually to 'fork':

import multiprocessing as mp
mp.set_start_method('fork')

This is done starting with HiCExplorer 3.6 version to support Python 3.8 on Linux and macOS. In the long run, this issue should be fixed.

https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods
https://bugs.python.org/issue33725