APE7 - A plan for NDData

This is ready for review. Comments are solicited until **Friday 14th November** after which a vote will be taken on whether to accept or reject this APE.

(note, this has been renamed to APE7 since there is already an APE6 proposed in #7)
I would advocate for these to be included by a mixin class unless someone can alleviate my ignorance about how the astropy registry works and if I could hook these into sunpys custom readers rather than say pyfits for fits io.