Improve prime OPS example script

The OPS library is focused on image processing, and as such it would be best if the front-facing example script did more than gimmicky operations. Rather, it should perform a really common and useful—but simple—image analysis workflow. We want something succinct that elegantly performs some powerful image processing.

We need to update both the `README.md` as well as the [using-ops tutorial](https://github.com/imagej/imagej-tutorials/tree/master/using-ops).