Progress bar: Add progress bar when simulations take a long time

[kevindavisross]

Could be as simple as displaying the counter of the number of non-rejected simulated values. Could have a delay - it only displays if the simulation takes more than 5 seconds or something.

[ellisonbg]

The vdom MIME type that works in JupyterLab/nteract would be a great way of doing this:

https://github.com/nteract/vdom