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)
Agreed, though it probably should be `np.asanyarray(nd.data)` so that if `.data` is already an `ndarray` subclass it is passed through as is.