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 think it is problematic to let the decorator fail is an ``NDData`` property is set, but the function does not support it. I image e.g. a function that shifts a spectrum from heliocentric to the reference frame of a star. All this function needs is the wcs (and maybe ``meta`` to record this shift in a keyword), but with this implementation is would also be required to take ``data``, ``unit`` and ``uncertainty`` arguments just because the decorator fails if not every attribute of the ``NDData`` is accepted by the function.