Integrate Jupyter notebooks into automated tests & Travis CI

[zaneselvans]

Currently we have no process for ensuring that our notebooks remain valid as the underlying codebase changes. In most cases this isn't a huge issue -- many of them are really scratch paper -- but it would be useful to make sure a few of them continue functioning:

• Notebooks we use in the integration of new data as it is released periodically
• At least one introductory / documentation notebook for new PUDL users/contributors
• Notebooks used to perform example analyses and data visualizations the PUDL data and analyses.

It looks like there are several plugins to PyTest that may be able to help achieve this goal, and get the test system to run specified collections of notebooks. nbval is the best looking option I've come across. Documentation here

[zaneselvans]

The nbval tox environment does most of this -- closing and replacing w/ a re-org of notebooks into travis / non-travis tested ones...