🌠 Add a nox session and GitHub workflow to test evaluation of completed notebooks

[namurphy]

It's probably too late for it this year, but one possibility for next time would be to:

• Create a Nox session to execute the completed versions of notebooks (perhaps with nbconvert). If the repository contains a requirements file that exactly matches the HelioCloud environment, we can have a pretty reliable check that the notebooks will work in that environment. ✅
• Create a GitHub workflow that would run the Nox session as a cron job and on pull requests.

Nox is nice because the configuration files are written in Python, which is magical 🪄! An alternative would be to use tox should we prefer to stick with .ini configuration files. 😅