* Comment tweak
* Directly print traceback
Since the shell.showtraceback is giving some issues
* Make InteracrtiveSceneEmbed into a class
This way it can keep track of it's internal shell; use of get_ipython has a finicky relationship with reloading.
* Move remaining checkpoint_paste logic into scene_embed.py
This involved making a few context managers for Scene: temp_record, temp_skip, temp_progress_bar, which seem useful in and of themselves.
* Change null key to be the empty string
