Add makefile recipe for testing Jupyter Notebooks

[bird-house]

Add makefile recipe for testing Jupyter Notebooks

[cehbrecht]

I suppose the notebook tests are optional ... so we don't need to add the requirements to the default environment.yml but may have an extra folder/environment like in birdy?
https://github.com/bird-house/birdy/tree/master/notebooks

[Zeitsperre]

I didn't realize that Birdy already had a few examples. I can borrow a bit of code from that to address this issue. The goal would be to streamline the testing of notebooks in the makefile so that users can easily run tests themselves. For the requirements, these would strictly be for development purposes so placing them somewhere away from base requirements makes sense.

We could also consider adding a few notebooks that could be integrated directly into the docs build, but that might be worth opening as a separate issue (see: https://pavics-raven.readthedocs.io/en/latest/notebooks/index.html).