Changing aperture photometry output to return astropy.table

This is the first draft for returning the photometry results in an ``astropy.table``.
I think the main question is what we want to return. Only flux, fluxerr, or something more, e.g. positions, or even the apertures (as metadata), etc.  

I'd find it simpler to add `xcen` and `ycen` as table columns, sort the table and print the first and last row.
But it's also fine as is if you prefer.