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!
We should have the eta coordinate from the heliocentric system, which means we need to know the observer-sun centre distance to do this transformation. This should be user specifiable some how, maybe through `SkyCoord`?