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)
@mwcraig - I like it up until you specify the convention.  What I meant before was that we should just list a few specific strings - I think the logic you're saying here, based on numpy function names is not terribly useful for anything other than ``'std'`` and ``'var'``.

See astrofrog/astropy-APEs#5 - I've re-written the description there to be clearer what I mean.

Note one other signficant change there: I'm saying it's a *requirement* that ``uncertainty_type`` be a string.  I think that's ciritical because the whole point is for it to be easy to check for it.