Evaluate grid PSF on a subset?

How can I run PSF photometry on a cutout of an image using a gridPSF model?

There's probably some clever way to wrap the `grid.__call__` to give it an offset, but I haven't come up with it yet.

The issue names PSF photometry on an image cutout, a gridPSF model, and grid.__call__, but no files or tests. Start by tracing the PSF photometry entry point and how grid.__call__ evaluates positions, then determine how an offset should be represented for a cutout. Done means the supported behavior is defined and verified for a cutout using a gridPSF model.

Written by the indexing model from the issue text.