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 need to look into the io stuff, the current SunPy implementation leaves some thing to be desired. However, this is not currently a part of our API and I would prefer it if it was not enforced in the base class.