Improve tests

[rlouf]

Tests not only allow to make sure the code is working as expected but they can also be used to make sure that the documentation and examples are up to date.

• Test the code examples in the doc
• Test the code in docstrings
• Run examples and notebooks to make sure they still work

Documentation should be quite fast to test so can run with the CI after every push. We might need to do a nightly run for the examples. Not sure how that would work with the data either.