Increase test coverage

[Pennycook]

Our test coverage is currently only around 89%, which may be hiding some bugs.

A simple way to compute coverage statistics:

python -m coverage run -m unittest
python -m coverage report

[laserkelvin]

Should probably consider adding a coverage badge as well?

[Pennycook]

Should probably consider adding a coverage badge as well?

Yes, good idea. It would be good to have this as part of the CI.

[Pennycook]

As part of this issue, we should also refactor bin/codebasin so that the functionality can be tested by unittest.