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)
Just to be clear, even if these are not implemented as a mix-in, you can override the readers/writers thanks to the registry. However, I do think it's a nice idea to factor it out into a mix-in.