I would like to suggest the following renames:

annulus_circular -> aperture_circular_annulus
annulus_elliptical -> aperture_circular_annulus

I've had comments that the current names are a bit confusing, and it would be better for all the functions that do aperture photometry to start with aperture.

An alternative would be to combine annulus_circular and aperture_circular into a single function that takes an optional r_in argument. If not specified, the aperture is cir cular, otherwise it's a circular annulus.

@kbarbary @cdeil - any thoughts on this?