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)
Suggested addition: 
"""
If present, subclasses should ensure numerical operations properly take into account and propagate units. Sub-classes could choose to connect this to ``data.unit``, in which case ``data`` should be a ``Quantity`` (or behave like it).
"""