Adding a handful of example plotting notebooks

[mezarque]

PR checklist

• [x] Describe the changes you've made.
• [x] Describe any tests you have conducted to confirm that your changes behave as expected.

Overview

This PR adds in a handful of plotting examples in Jupyter notebooks in the plot_testing directory. It also makes adjustments to the matplotlib default styles in style_defaults.py that bring us closer to the final values (but these will need to be updated again later).

Testing

All of the notebooks seem to work fine and generate relevant plots correctly.

Next

After this PR, there will be a separate PR that updates the base gradients, styles, and figure sizing to match the finalized version of the style guide.

[review-notebook-app[bot]]

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[mezarque]

Thanks for this feedback! I created these issues to deal with in future PRs:

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