Add support for arbitrary airfoil sections

[markstock]

There is a simple and standard format for airfoils (wing sections) which we should support, see https://m-selig.ae.illinois.edu/ads/coord_database.html or http://www.airfoiltools.com/ for many examples. It would be nice if our json format would support either a file name or the raw data itself.

Upon loading, we'd need to join the trailing edge points (they are often non-zero thickness), and also compute spline paths for the segments to allow us control over panel size.