APE7 hangout updates

This includes the updates discussed in the hangout with one change: the proposal to implement a Quantity/NDData subclass or, if that turns out to be unfeasible, a subclass that uses ``Quantity`` for the ``data`` and ``unit`` (and that would permit tying into ``Quantity.uncertainty`` or ``Quantity.mask`` in the future).

I'd recommend merging this in, then I'll ping the interested parties to comment on the PR on astropy/astropy-APEs