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)
Following the main thread: Is this statement too weak? For gridded data, should it not be a requirement that `data` has a `shape` and that it can be indexed/sliced?