Feature/auto testing

[rbs333]

PR contents:

• adds .github/workflows/tests.yml which executes on push to main and pull_request
• test uses nbval as opposed to treon. Nbval build into pytest and allows you to skip certain cells easily which is advantageous for testing environment.
• style tweaks to notebooks discovered while testing

this is a basic v1 to get us moving with testing on these notebooks to make sure they remain up to date and we can catch if something goes stale.

[tylerhutcherson]

Left a small comment, otherwise looking good.

[justin-cechmanek]

Looks like a good start to cleaning up our notebooks.