Add code coverage analysis

[ErikBjare]

Another improvement to the CI pipeline.

We don't have much in the way of tests, but we do have the auto examples which run a fair bit of the code. It'd be nice to know exactly which code is somewhat tested in CI, and which isn't (and when new code is added, how well covered it is by tests/examples).

This should be easy enough to do with coverage.py.

[JohnGriffiths]

Agree- the examples have a very test-like function but that isn't explicitly documented. Coverage tracking would be great to include

[ErikBjare]

I added coverage for *.ipynb notebooks in my thesis repo the other day, but I'm not sure how to do it for the auto_examples...

I gave it a shot in #88, but it's not ready yet.