Add correct psfFlux PSF scaling with aperture correction to Postage Stamp

[wmwv]

The PSF subtraction is intentionally naive. But perhaps not sufficiently so. A new user might be misled to think that it's showing a reasonable way of getting the full contribution of a source.

Eventually add on how to do this right:

 psfIm = exposure.getPsf(s.getCentroid()).computeImage()
 psfIm *= s.getPsfFlux()
 exposure.maskedImage[psfIm.getBBox()] -= psfIm
AND ACCOUNT APERTURE CORRECTION

Showing this would require adding (1) querying the coadd catalog and (2) doing the aperture correction right.

[wmwv]

Move to DC2-analysis repo. Low priority