Add notebook extensions for code formatting

[Peter9192]

I came across this blog post on notebook extensions. I think particularly point 2 is nice. It adds a button for autopep8 formatting. Perhaps we could use this, or something similar, in our notebook environments.

See the notebook extensions page on RTD. Perhaps also of interest: Code prettify, issuing a call to yapf.

[sverhoeven]

This extension is for the classic notebook env, we are using JupyterLab on https://jupyter.ewatercycle.org.

For JupyterLab there is a similar extension called jupyterlab_code_formatter, it can be installed instead.

[bouweandela]

Auto formatter notebook plugin based on yapf: https://jupyter-contrib-nbextensions.readthedocs.io/en/latest/nbextensions/code_prettify/README_code_prettify.html

And imports formatter notebook plugin based on isort: https://jupyter-contrib-nbextensions.readthedocs.io/en/latest/nbextensions/code_prettify/README_isort.html

[bouweandela]

For JupyterLab there is a similar extension called jupyterlab_code_formatter, it can be installed instead.

This also supports yapf and isort. It even supports black, so we could also decide to use that instead.

[bouweandela]

We decided to use black -l 79 -t py36 for now. A suggestion from @nielsdrost for using black from a notebook was: https://github.com/drillan/jupyter-black