update notebooks

[M-Lampert]

This PR updates the Jupyter notebooks so that they work via Google Colab by default.

• Add prerequisites section and pathpyG installation.
• Fix FileNotFoundError in temporal_graphs.ipynb
  @IngoScholtes do you know if the name is wrong or the file is missing?
• Fix TypeError in paths_higher_order.ipynb that is due to the recent merge
• Come up with a solution to missing files when using Google Colab (temporal_graphs.ipynb, dbgnn.ipynb and paths_higher_order.ipynb)
  Since only the Notebook is opened via Colab, the accompanying files from the repository are not copied over and, thus, an exception is thrown. This could be either fixed by using datasets from Netzschleuder instead or by manually downloading the files from our GitHub Repo in the notebook if they are not available. @IngoScholtes what do you think is the better option?

[IngoScholtes]

Regarding rhe FileNotFound error ... this is due to the reading of a data file that is currently not in the repository. I'll have to check whether we can push it.

I guess it may be best to not depend on any data files, as this would also solve the second issue.

[M-Lampert]

While we are at it, it might anyway be a good idea to clean up the docs/tutorial/ and docs/data directories.
@IngoScholtes do you still know the sources of these files? Are they from Netzschleuder? Then it might be an option to download them as part of the Notebook.

But if they are from Netzschleuder, it might be easier to first implement the new logic for reading data from Netzschleuder since you mentioned once that you want to change that to downloading and reading the available .csv files instead of the graph-tool format.