Start to style the PyGMT Basic Mapping NB

[calum-chamberlain]

Styling for the PyGMT notebook that will replace 4A - do not merge yet, WIP.

[calum-chamberlain]

To do:

• [x] Update the headers for all notebooks to match the PyGMT ones
• [x] Check style
• [x] Get tests running

[review-notebook-app[bot]]

Check out this pull request on 

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[calum-chamberlain]

Tests pass locally and autopep8 has been run for code-style. I think now we need to review these for actual content and style!