Add the Lorenz differential equation notebooks we use in screenshots.

[jasongrout]

This is a simplified notebook from the poster-child example for the notebook, the ipywidgets Lorenz demonstration. I simplified it slightly to make it easy to take a screenshot containing both the equations and the interact.

CC @ellisonbg

[jzf2101]

@jasongrout don't we need lorenz.py?

[jasongrout]

Yes, it's here.

[jasongrout]

https://github.com/jasongrout/jupyterlab-demo/blob/2827cb897abbd812d51799811208b724a84fbc27/notebooks/lorenz.py

[blink1073]

Thanks!