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 worry a bit about the no restrictions aspect of this. The ND part does imply that the data object is indexable. I do feel that the greatest use of this class is when the data attribute can be guaranteed to be able to be cast to a ndarray. But if that can be done in the concrete class, that's ok.