Implement testing for example notebooks

[JoYvBa]

Write tests for notebooks to make sure that they always work with the newest version of mibitrans

[JoYvBa]

Having looked into it a bit, the most common way to do this is with pytest-nbmake. And while a simple test to see if the Notebook does not throw any error should be quite simple, things get more complicated really quickly when testing for functionality or output. It is possible to included the tests inside the notebook itself, but since it will serve as documentation, we do not want it in. I could not quite figure out how to do it with an external test script, like we have for the rest of mibitrans. @raar1 Do you have any suggestions on this matter?