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)
It's a minor point but I suggest we avoid using "pixel space" and say something like "relationship between input and world coordinates" as this can be useful if the inputs are not in pixel space.