Notebook-based tests should be converted to normal Python test scripts

[mhvwerts]

The direct use of Jupyter Notebooks with pytest is not very practical. The required pytest_notebook plug-in is not compatible with all versions of pytest.

Those 'example & test' Notebooks that are sufficiently mature should be used as the basis to normal Python test scripts, which are then included in the tests folder. Once this is complete the dependency on pytest_notebook can be removed.

The Notebooks themselves will remain as examples, and will be evolve separately. In general, it will not necessary to further change the resulting test scripts, because they already test what needs to be tested. This should, of course, not stop us from continue adding further (normal Python) test scripts to enhance testing coverage.