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)
How do we ensure proper sliceability? That it has a __getitem__ method isn't sufficient (a test slice with index of the dimensionality implied by shape?). We may want to note that early implementations may not be able to enforce this as strictly as needed.