APE6 preliminary review

The topic of uncertainties might be a subject for another companion APE.  I think it's worth having as a core feature and could be useful in writing functions that use an `NDData`'s uncertainties effectively if/when available.  Right now there's just not a lot of clarity on how that would work except that there would be a place to look for it.  There already is the beginning of something like this in the form of [`NDUncertainty`](https://github.com/astropy/astropy/blob/9f9327a20f4c57d7df04ad3542d88340508cff73/astropy/nddata/nduncertainty.py#L31) which I think is what's in mind here.  That there will/is some standard interface for representing uncertainties as well--just one that is also not necessarily useful outside of application-specific subclasses.