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.
Don't use `\` for continutation, you can use `()` on both sides of `=`

```python
(a, b,
     c) = (d, e,
             g)
```