HEALPix manipulation

A HEALPix is a way of dividing a sphere such that every point has an equal area. This library gives utilities for working with the NESTED scheme, which is optimal for neighbor checks and subdivision. A good deal of code has been taken from the cdshealpix crate, but the API has been reworked to be cleaner and easier to use.

The majority of the implementation logic was forked from commit 5e58d17 of https://github.com/cds-astro/cds-healpix-rust.git.