Add Jupyter notebooks

Add Jupyter notebooks in the scripts folders of each case. For this, rearrange the Python scripts a bit. In particular, move the definition of the plot routines to new files plotroutines.py, import them from the notebooks as well as from pol.py and pot.py. Make the folder naming consistent in terms of hyphens and underscores. Add a file styles.py in a new subfolder common that defines the line style and the color for each place and method.

GitLab is already able to display Jupyter notebooks, although only a static view. Once we publish the repository, my plan would be to have the notebooks executable via https://mybinder.org/.