Move "is_initialized" flag for 3D scene from server to client

[afullerx]

This PR resolves #3339 by moving the existing is_initialized flag from scene.py into scene.js. This way, each client maintains its own copy. 

I also removed is_initialized from scene_view.py. It looks like it was copied over from scene.py, but didn't serve any purpose since there are no methods for creating objects. 

[falkoschindler]

Thanks for the pull request, @afullerx!

Moving the is_initialized flag to the client is probably a good idea.
But if I understand correctly, you changed the behavior from "not running any methods before initialization" to "not creating objects before initialization". I'm not sure if this is a valid simplification or if it can break something else. I haven't yet come up with a counterexample though. 🤔

[afullerx]

I considered this, but decided it was OK since most of the methods begin with if (!this.objects.has(object_id)) return. So, they are effectively prevented from running until the scene is initialized. The only two exceptions are move_camera() and resize(). I assumed this is because they can run at any time, since they only affect the view of the scene and aren't dependent on its contents.

[falkoschindler]

Yes, that makes sense. 👍🏻