Clean up first part of vortex notebook.

[ketch]

I didn't go over the whole notebook, but I made some cosmetic improvements to the early part. Note that, due to size and a desire for clean diffs, we store notebooks without output on Github.

There are some formulas in the code that don't match the formulas in the text. I didn't try to fix them since I don't know which is right and which is wrong, but they should be corrected.

Note that num_output_times is not the number of time steps: https://www.clawpack.org/pyclaw/output.html#when-output-is-saved-written

The term "grid size" is potentially confusing since it's often used to refer to $\Delta x$.

[ketch]

Also, I wouldn't refer to the plots you've created as "schlieren plots", since you're using a red-white-blue color map.

Since there are no shock waves in this problem, it's probably not best to use a schlieren plot.