Issue #19: Release GIL when executing ndiCommand.

[MattClarkson]

Release GIL around ndiCommand - not tested yet.
Will test, when back in the lab.

[thompson318]

This looks good to me. Will enable multithreading and I don't think introduces any new bugs in ndicapi. A couple of things to think about.

1. This will allow the calling application to make multiple simultaneous calls to the same tracker, which could cause problems. This is probably best managed by the calling application, so add a note to the function description.
2. I'd like to avoid our version of ndicapi diverging from https://github.com/PlusToolkit/ndicapi. The idea is that 2github-actions just adds the infrastructure for building wheels and publishing to pypi to PlusToolKit's master branch. So I suggest adding some documentation then creating a pull request to https://github.com/PlusToolkit/ndicapi/tree/master. They're generally good at responding and may also pick up on any other issues with the PR. If that works then I will update our version and rebuild.