Add lessons on scripting and OOP

[ethan-pierce]

Adds a new lesson "Scripting and the Shell" to the Python lessons folder. This notebook goes over the basics of shell commands, text editors and IDE's, and includes a short series of exercises on building and running a Python script. It is intended to directly follow lessons on hillslope diffusion and writing functions.

Also replaces the yet_another_oop lesson with a more direct follow-up to the 1D diffusion lessons. This new OOP lesson covers the same material, but uses 2D diffusion as a working example.

[mcflugen]

@ethan-pierce One thing I use all the time when working in the shell—and wish someone had told me about earlier—is brace expansion (e.g. cp foo.py{,bak}). Maybe this is something that could be listed under "Random tips and tricks"?

[ethan-pierce]

Thanks @mcflugen, added to random tips!