Better unit tests for model outputs

There are several issues with the outputs which have gone under the radar. I think one reason for this is that the test coverage for the outputs is poor, and the example models only use a small subset of the tests.

We need to add unit tests for every output, and specifically check:
- whether the output runs successfully with an example model (e.g. the `default` model), or several example models
- check that the column titles are as expected
- a few other checks where relevant (e.g. no zero or nan values)