APE5: Coordinates Subpackage Plan

This APE provides an outline of a proposal for how to move forward with the coordinates subpackage.  It is an attempt to compromise between the various suggested approaches in a way that I hope will be acceptable to all parties, as well as a plan for how it might be implemented. 

I think it also will make it easier for multiple people to work on `coordinates` at the same time.  This has been a problem in the past, where a single person (in most cases, me) would roadblock parallel work because they were making changes that would have a variety of important consequences for other efforts.

Note that this APE is tied closely with astropy/astropy-api#12 - that outlines the actual proposed API, without which this APE is rather abstract.