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 thought I had originally written precisely this but it may have evolved over time. But yes, I think that we should require that ``data`` has a ``shape`` and can be sliced.