Run tutorials as tests

[dalonsoa]

WSIMOD includes several tutorials that could very well serve as integration tests if run as part of the test suite. Initially the goal would be just to check that they run without errors.

A further refinement that could be implemented would be to save the outputs of running the models in the tests folder as the "truth" and then, when running the tutorials, compare if the actual outputs agree with the expected ones. But that requires a bit more work and will need to be done after #24 since at the moment no output files are produced.