lecture 4 -- speedup and variable renaming

[fonsp]

I renamed S to sim, to avoid confusion with solar insulation later.

The plot_state function called as_png, but later we call as_svg on the result. This combination made things quite slow -- it's probably my fault that both were in the code, sorry!

The simulation is now very fast! You can get about 10 frames per second.

PS you might want to merge lecture23 -> master ?

[fonsp]

The diff is quite large, but this is mostly because I removed some dependencies between cells, so Pluto saves them in a different order.

[hdrake]

@fonsp my hero