WIP: Implement spherical harmonic transforms on only observed portions of the map

[jmert]

The goal is to reduce the computation time spent on performing the spherical harmonic transforms for maps which cover only a small fraction of the sky. The BICEP/Keck (BK) analysis has long relied on a customized version of synfast which can output such "cut-sky" maps to save time (and disk space) on the alms → map transform, but the same ability for map → alms is in principle possible as well.

This will be especially important to make spherical harmonic transforms of the BK maps economical when compared to the existing practice of using FFTs. The maps cover a very small fraction of the sphere, so doing full-sky transforms at all times is incredibly wasteful.

• Add utility function to scan a given full-sky map for the list of rings which have observed pixel values. (Observed defined as non-zero and non-NaN.)
• Add an interface to provide a map with an explicit pixel list. (The true cut-sky input.)
  • Will require a transform back and forth between cut-sky to cut-rings format.
• Add an interface to request a subset of the sky be synthesized from a given set of alms.

(Replaces #3 which I managed to close and can't open again)