APE6 preliminary review

> No function in Astropy or in affiliated packages would be required to be able to handle generic ``NDData`` objects.

But having `NDData` is only useful if there are lots of functions that can handle `NDData`.
So in practice most functions in `astropy.image`, `astropy.convolution`, `photutils`, ... should be changed and the decorator proposed below added, no?
Am I misunderstanding this sentence or is it indeed misleading?