Where should the PSF be centered for sending to SFFT?

[rknop]

Right now, PipelineImage.get_psf does:

        wcs = self.image.get_wcs()
        x, y = wcs.world_to_pixel( ra, dec )
        ...
            self.psfobj = PSF.get_psf_object( psftype, x=x, y=y,
        ...
        stamp = self.psfobj.get_stamp( x, y )

I think, for purposes of SFFT, we want to have PSFs centered on a pixel. To accomplish this, we should be sending floor(x+0.5) to both get_psf_object and get_stamp.