Q rings, q indices, number of pixels in each q ring

This module will compute the Q values for all pixels in a shape specified by
detector_size and find the indices of the required Q rings. Count the
number of pixels in each Q ring.
I think the problem with this is it will give you regions labelled `[1, 2, 4, 6, 8, ...]` when I think you want `[1, 2, 3, ..]`