examples: Add a notebook with a toy variable-z topography implementation

[EdCaunt]

I made a notebook with a simple implementation of a toy variable-z curvilinear acoustic model with a cosine hill. Still WIP, needs tidy-up and explanations, but lmk your thoughts

[review-notebook-app[bot]]

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[codecov[bot]]

Codecov Report

Merging #1900 (e18da51) into master (f80847e) will not change coverage. The diff coverage is n/a.

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[rhodrin]

Overall looks good. Just added some comments above that I think would help make it more easily digestible.