subsampling and dual image mode

Subsampling and dual image mode won't work right together. Subsampled versions of the detection image and weight map don't write out, but a subsampled measurement image and weight map will write out. Sextractor won't use images of consistent size then.

On top of this, the detection PSF is adjusting the offset to be resampled. Not doing this would be more consistent with the current detection image and weight behavior. But keeping it as resampling would be the right thing to do if the detection image and weight were subsampling.