Add a Google Colab badge to example notebooks

[mberz]

Adds a badge which opens the respective notebook in a Google Collab environment.

Implemented analogous to the mybinder.org support.

As a sidenode: Google Collab does not allow to install requirements via a conda environment as far as I can see. Yet, the main benefit is easily storing the notebook in a Google drive folder.

[f-brinkmann]

Should we really do this? This means that people who open a notebook on Google Colab get an error when trying to run the first cell containing the imports. If they blame it on pyfar, this might prevent them from using it any forther.

This could be solved ba adding !pip install some_package commands to each notebook. Do we want to do this?

[mberz]

Indeed, adding a statement on this is something we should do.
I would however very much like to emphasize that the progress you make when running a notebook on binder is lost once the session times out. So I especially would see this as a very nice thing to have especially for the OER.

The additional comment including the requirements would also be highly beneficial for everyone who wants to execute the notebooks in their local environment.