convert license headers to SPDX

Expand loop example, fix logic.py bugs, solve.

A new version of the loop-example also includes a loop with a
function call. Some tweaking in logic.py is necessary to detect
the key linear sequences, but the problems all solve pretty quickly
now, which is a huge improvement on their original times.

Introduce loop-example.

This was a running example from a previous demo of graph-refine.