Funtracks

What's Changed
* Bugfix: Check for empty solution tracks when getting next track id by cmalinmayor in https://github.com/funkelab/funtracks/pull/6

New Contributors
* cmalinmayor made their first contribution in https://github.com/funkelab/funtracks/pull/6

**Full Changelog**: https://github.com/funkelab/funtracks/compare/v1.0.0...v1.0.1

Initial release, mostly copied from motile_tracker v3.0.0.

One functional change: We no longer prompt the user when attempting to make invalid changes, especially when adding a node to a track that has an "upstream division". Instead, we just reject the change with an error message.
Additionally, some typng changes made to satisfy mypy.