Huge bug with multiprocessing

[alexreg]

This program make the invalid assumption that mutated global variables (like BACKBONE_GROUPS) will have up-to-values in child proceses. This is very much not true, an explicitly stated by the Python docs! It causes the program to fail to produce any useful output on MacOS, for a start, since BACKBONE_GROUPS and MAIN_HAPLO_GROUPS are empty (default values) in child processes of the Pool.