document specifically how spiner handles setRange

[Yurlungur]

PR Summary

I believe there was some confusion with setRange about the order that indices in setRange are set correspond to. They're in reverse order from the index order in, e.g, a call to interpToReal. This is now more carefully documented. I also add a test demonstrating this use case.

PR Checklist

• Code is formatted. (You can use the format_spiner make target.)
• Adds a test for any bugs fixed. Adds tests for new features.
• If preparing for a new release, update the version in cmake.