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 would be fine with @eteq edits.   I could also imagine adding something like 'likelihood'  and 'probability_distribution' but I'm not sure I would be at a point to put them down in an APE in such a specific way, but it might be worthwhile to point them out as possible ways to specify an uncertainty.