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)
This might be better worded as "the effect of slicing on attributes presumably returns a similar object, e.g., for wcs, it returns a new WCS appropriate to the sliced data attribute."