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.
```python
q_inds[q_inds % 2 == 0] = 0
indx = q_inds > 0
q_inds[indx] = (q_inds[indx] + 1) // 2
```

Should do what you want.