Add an option allowing use without a multiprocessing pool...

[valrus]

[Spleeter] - Add an option allowing use without a multiprocessing pool

Recreating this PR from #231 by request from reviewers to trigger CI.

This PR adds a keyword argument allowing a Separator to be used asynchronously within a multiprocessing-created subprocess (by bypassing its multiprocessing.Pool and doing its work in the main process). Without this option enabled, attempting to do so will fail with an error "AssertionError: daemonic processes are not allowed to have children."

How this patch was tested

I'm working on a tool that calls separate_to_file using Pool.apply_async. Prior to this patch it failed with the AssertionError above. After applying the patch and passing multiprocess=False to the Separator constructor the separation works as expected.

Documentation link and external references

Info about the exception in the Python docs: https://docs.python.org/3/library/multiprocessing.html#multiprocessing.Process.daemon

Here's a SO example of someone encountering the same exception in a similar scenario: https://stackoverflow.com/questions/51485212/multiprocessing-gives-assertionerror-daemonic-processes-are-not-allowed-to-have

It seems that this problem could also be avoided by using a ThreadPool but I understand spleeter to be fairly CPU-bound so threads aren't a great solution. However if spleeter does most of its heavy lifting in its own subprocesses, it might work to use a ThreadPool instead. I'm willing to try it if you'd prefer.