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Fix crash on macOS with Python 3.8 and above

Starting with Python 3.8, CATCH crashes on macOS. It is caused by a
behavioral change between Python 3.7 and 3.8, in which multiprocessing
in Python 3.8 on macOS switched to spawn processes rather than fork
them. Apparently forking processes in macOS can cause crashes, but CATCH
with older versions of Python has not experienced those issues.

Parts of CATCH --- especially, the pools in the `probe` module --- are
written to follow the fork behavior, where child processes inherit
memory from their parent. When spawning processes, those child processes
do not inherit memory, and CATCH crashes because the global variables
are not accessible by the child processes.

This commit introduces a quick fix: telling the multiprocessing module
to fork processes by default, rather than spawn them.

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