Adds basic skymap/fits utilities

This adds the basic utilities for generating skymaps. In future, these can be integrated into the result object itself, but for now I'm just adding the core functionality.

Note, this may produce a large pile of output saying "Invalid ring index", this is a known issue and is being fixed: https://github.com/astropy/astropy-healpix/pull/106

Partially closes #123 (closed)