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 this make more sense as a `(N, 2)` array, rather than `(2*N, )`.  That way you can do

```python
for i, (start, stop) in enumerate(q_ring_val):
    print("the {}th ring goes from {} to {}".format(i, start, stop))
```