DEV: Add nbval to CI.

[rossbar]

Closes #51 .

Add a basic nbval workflow to the CI, borrowing heavily from @MridulS 's recipe in nx-guides. Currently only smoketests the notebooks with pytest --nbval-lax. More rigorous testing (i.e. comparison with saved outputs) can always be explored in followup PRs.

A couple of potential improvements:

1. Find a better way of programmatically excluding certain notebooks from the test suite (see TODO), preferably via conftest.py or similar mechanism
2. Add data caching to the workflow
3. Add a pytest and nbval as testing requirements and update the contributor guide with a quick note on how to run locally.

[bsipocz]

Any reasons to not add windows, too?

[rossbar]

Only because I expected it would require special windows-specific handling to make it work... I just tried it and it seems that's the case, see: https://github.com/numpy/numpy-tutorials/runs/7309090446?check_suite_focus=true

My vote is for incremental adoption: get linux/macos in sooner since they're working now, and open a follow-up issue for anyone who wants to figure out what magic combo of workarounds is necessary to get it to work on windows :)

[bsipocz]

I'm still not 100% happy with my CI solution here: https://github.com/NASA-NAVO/navo-workshop, but I think adding tox helps a lot with point no3 in your list, and it also helps to manage and run the same command through the different CI providers and by contributors.

[rossbar]

I'm +1 for a tox.ini (or whatever testing config file) as well. I'd also vote that for that to be done in a follow-up PR so that it's self-contained, but if you'd like to add it here please feel free to do so!

[bsipocz]

@rossbar - it's your PR, so whatever works for you sounds good. I can put doing some incremental stuff for this as part of my scipy sprint, but no promises that I actually manage to go through that list.

[rossbar]

I'm going to go ahead and put this in as-is to get the additional testing in place. I'll open issues for the follow-up tasks that were identified above. Thanks @bsipocz !