pytest runner for Jupyter notebooks

[nickponvert]

It would be useful to have Jupyter notebooks run as part of pytest for several reasons:

• Code that is included in the notebook cells could be tested to see if it still evals
• Figures produced by the notebook running after the change could be diffed to see if a change makes plots look different, even if the code still runs. Attached is a notebook diff produced by https://app.reviewnb.com/

In this example I actually changed the code in the notebook, which produced a change in the figure output. However, we could use this to test if there is a change in plotting output due to a change in any of the dependency code.

[nickponvert]

This exists: https://github.com/computationalmodelling/nbval

[nickponvert]

Also relevant: https://www.cityscience.com/blog/ci-for-jupyter.html

[nickponvert]

In the works from the same people as ReviewNB: https://github.com/ReviewNB/treon