Create test that runs the notebooks once every month

[martinvonk]

Allright I did a few things:

• Create a test to run the notebooks with _sol. This can also be run locally via pytest .github/workflows/test_notebooks.py
• Update some of the notebooks such that they run again.
• Remove 3 old .ipynb_checkpoints

Note that I ignore a few notebooks:

• bugs notebook (because that one aught not to run :)
• wm notebooks (couldn't open them)
• s3 and s4 notebook (couldn't open them)