Jupyter notebook guide enhancements

[arfon]

In #23 I've created a first initial pass at a notebook style guide. There are a few known issues I'd like us to address:

• License statement: We should include a license statement (suggest CC-BY) at the bottom of each notebook.
• Table of contents: Currently, the example notebooks we link to have Tables of Contents created in a less-than-optimal way (lots of custom HTML). Currently JupyterLab doesn't seem to support native Markdown lists with section links - we need to find a workaround here to https://stackoverflow.com/questions/50467064/unable-to-use-anchor-tags-to-create-table-of-contents-in-jupyterlab.
• What to do about custom requirements?: We could ask people to define a requirements.txt file but currently we wouldn't use it anywhere...