Overhaul regression testing and plots

[LiamPattinson]

pytest-regtest is not a widely used pytest plugin, and its implementation of snapshot testing is fairly non-standard (although it does seem to handle NumPy/Pandas/Polars...)

I worry that teaching regression testing with matplotlib could lead to bad habits forming, e.g. testing the plot of some calculation rather than testing the results themselves, which could be dangerous.

I would lean towards teaching syrupy as the standard solution, and dropping the matplotlib part. Textual data would be easiest here. For NumPy etc, we can point people towards pytest-regtest, snaptol, or just handling it manually at the end.

[LiamPattinson]

Had a chat about this, and I believe the plan is now to teach manual snapshot testing to get the concepts across, and then teach snaptol as a tool to automate the process and make it less error prone. @avadean let me know if you need any help with this!