Better management of scripting during interactivity

[matthewturk]

At present, we use some threads and whatnot to get at the underlying data and scene objects in yt_idv if we execute it interactively from a script.

It would be awfully nice to make this easier to use.

Furthermore, the last few times I have tried with this in ipython, the threading setup was not well-suited to it, and it resulted in a lot of hangs and delays.