Add helper API call to reset the application to a clean state

[rosteen]

This adds the ability to call viz.reset_app() to completely reset the Jdaviz application to a clean state. The only outstanding issue that I'm aware of is that after resetting, creating new subsets continues to iterate from where they left off for labels and colors:

Screen.Recording.2024-11-07.at.4.45.01.PM.mov

cc @eteq

[eteq]

Happy to see this! out-of-band I had asked about this functionality, with a use case in mind of wanting to clear out large files from memory and reset various app details without re-creating the helper object (so that I don't have to re-show it, among other reasons).

It looks like this fully addresses my specific use case even without the subset fix. FWIW, I think if that's going to be a significantly heavier lift to do the subset part, it might be fine to merge this functionality with that limitation (as long as it's noted in the docs) and have a longer-running issue to address that last bit.