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
bin_starts = np.arange(nbins) * step
q_ring_val = np.vstack([bin_starts,  bin_starts + delta_q]).T.ravel()
```