Expanding on the addition of docstrings in code, this adds python module API docs to sphinx documentation.
A developer can preview what this might look like by running this locally:
```
tox -m docs
```
The option `-W` is added to the tox env so that warning messages are considered errors.
Adds doc generation using the above method to the test GitHub workflow to catch documentation problems on PRs.
Some docstrings have been reformatted and fixed to satisfy errors generated with the above setup.
Previously the only documentation for how to run tests was the
implementation in the Travis configuration file. Here we add
instructions as well as move development dependencies to an easily used
and shared (with Travis config) separate requirements.dev.txt file.
