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_ring_vals = np.cumsum(np.r_[0, np.tile([q_delta, q_step - q_delta], n_qs)])
```