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)
@astrofrog - Should this really apply to all properties? I don't think mask shares this ambiguity. Uncertainty also seems like a candidate for allowing setting.

Also, if we want to stick with the ``meta`` set and get provided by ``astropy.utils.metadata.MetaData``, then meta will also be settable.