Astropy Proposal for Enhancement 23: Removing data storage (representations) from coordinate frames (APE 23)

Following the rationale presented in APE 5, coordinate frames in astropy.coordinates currently store metadata used to construct the frame (i.e., for transforming between frames) and may also store coordinate data itself. This duplicates functionality with SkyCoord, which acts as a container for both coordinate data and reference frame information. We propose to change the frame classes such that they only store metadata and never coordinate data, superseding this aspect of the implementation in APE 5. This would make the implementation more modular and performant, remove ambiguity for users from having nearly duplicate functionality with slightly different APIs, and better satisfy the principle of Separation of Concerns.