Harmonic polynomial transforms on the disk, sphere, and triangle

This introduces the first iteration of the public API, consisting of computational kernels and driver routines for the harmonic polynomial connection problem and FFTW-based synthesis and analysis on tensor product grids. There are two examples, illustrating the spherical harmonic addition theorem, and the integration of holomorphic functions on the disk. Preliminary work on spin-weighted spherical harmonics is included, as well as support for components of a vector field in the tangent space to the sphere in spherical coordinates.

This is a joint effort with @dawese. Thanks to bug reports by @dlfivefifty and @ahbarnett.

Happy transforms!