The Proposed Coordinates API

This PR will contain all work on the coordinates API. The API tests are in a separate PR, and regular tests pertaining to various classes in the coordinates framework will be introduced in this PR.
Comments are very much appreciated!
Well, I was planning to do something about `HelioGraphicCarrington` some 70 or so commits ago...and then we decided to subclass `HelioGraphicStonyhurst` instead. This `__init__` is a remnant of that era. :D