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 think as long as the `.data` attribute implements an `__array__` method then that can be used to get an array. I am not so convinced by the need for `NDData` itself to implement this, obviously specific subclasses could.