Check why nbval works not for some cells

[danielfeismann]

With #359 we introduced a validation for the notebooks that works as documentation.

Within input_models.ipynb and result_models.ipynb the output of some cells are currently ignored (done by # NBVAL_IGNORE_OUTPUT). For some of these this does not seem necessary but for some reason the validation fails.

• check why it is failing
• fix it :)