Add files for testing purposes

[isabela-pf]

In order to run tests, we need some materials to test with. This PR adds:

• save-load-arrays.ipynb(originally at NumPy Tutorials)
• the NumPy Tutorials license (must be distributed with the notebook)
• x_y-squared.csv (courtesy of @steff456!)

all in a notebook directory until I get feedback on an appropriate location (or branch) to add these changes.

Thanks in advance for any feedback and review! 🌻

[trallard]

Left a comment and pushed a couple of changes to fix syntax issues as well as to clear the outputs