Error out when attempting to use multithreading on MacOS

[susannasiebert]

When running any of the pVACtools with multithreading (-t) on MacOS, all but one forked thread will be killed with:

The process has forked and you cannot use this CoreFoundation functionality safely. 
You MUST exec(). Break on __THE_PROCESS_HAS_FORKED_AND_YOU_CANNOT_USE_THIS_
COREFOUNDATION_FUNCTIONALITY___YOU_MUST_EXEC__() to debug.

The pVACtools run will still complete successfully, but all of the prediction calls that were scheduled for killed threads will not be made so a lot of predictions will be missing from the final output.

I tried various workarounds but wasn't able to get it to work. I put in a ticket with pymp-pypi, which is what we use for multithreading. In the meantime, this update will prevent users from running the tools in with multithreading on MacOS.