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.
but delta_q is 1 in the test, but you are getting back zero width bins.  You should add the tests

```
np.diff(q_ring_val, axis=1) == delta_q
```

and

```
step_q == np.diff(q_ring_val[:, 0], axis=0)
```