Streamline testing workflow

[awmacpherson]

Is your feature request related to a problem? Please describe. Currently, testing is carried out manually in multiple stages (run pytest, run notebook locally, run notebook on mybinder.org). This is tiresome and can be inconsistent.

Describe the solution you'd like This process can be streamlined and made reproducible by:

• Scripting the notebook test (using e.g. nbconvert) and running it in an isolated environment. Provide a temporary directory where it can create its output files and validate that the files are created.
• Create orchestration scripts for the notebook test and pytest invocations using e.g. make or tox.

[awmacpherson]

I've now created a notebook test and bundled it together with the unit tests in a tox configuration. All tests can now be run in multiple virtual environments using tox.

Future enhancements: clarify desired notebook behaviour and add tests for that.