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)
@hamogu - maybe we could make all this parametrizable in the decorator, e.g. you could say that you don't care if there is anything else that is not needed by the function, or that it can ignore specific attributes, etc.

Having said that, the way this decorator is designed, it would not work if the function takes *only* wcs - it would need to take a ``data`` argument too.