Doctests

[znicholls]

Is your feature request related to a problem? Please describe.

At the moment docstrings are untested.

Describe the solution you'd like

Enable sphinx's doctest extension, see here.

Describe alternatives you've considered

Use some other solution, don't test the docstrings or don't have docstrings.

Additional context

We've got sphinx now, this shouldn't be hugely painful.

[rgieseke]

Some neat documentation ideas in here (not sure if applicable ...) https://simonwillison.net/2018/Jul/28/documentation-unit-tests/