GriddedPSFModel evaluated PSFs with the same input flux have very different output fluxes

This has been tested with a GriddedPSFModel created out of a set of webbpsf Roman WFI apertures, using the sample code below. Note that instead of using webbpsf directly to create the PSF grid, instead the grid is created image-by-image and put into an NDData array after creation. This is because webbpsf currently does not preserve parity for even oversamples, and the simulator that I'm using these models for requires odd parity to build its EPSF models.

Start by reproducing the reported flux discrepancy with a GriddedPSFModel built from the webbpsf Roman WFI aperture images and the sample setup referenced in the issue. Trace how equal input fluxes are evaluated and compare the resulting output fluxes; done means the model produces consistent output fluxes for PSFs with the same input flux.

Written by the indexing model from the issue text.