make metacal more generic

[esheldon]

We can easily make it work for psfs and images with different pixel scales.

1. deconvolve image by pixel
2. deconvolve psf by its different pixel
3. deconvolve image by pixel-deconvolved psf
4. do metacal shears
5. reconvolve image by original pixel and chosen pre-pixel psf

[esheldon]

we should probably add a check that the psf pixel scale is not larger than the image pixel scale by more than a tiny amount (which can happen due to noise in the wcs)

[esheldon]

this can wait until after 2.0 comes out.