
ppSim development work still needed:

* save sources in output files
* load input image

* optionally apply detrend data (bias, dark, etc).  when adding data
  to a loaded image, we only need to inject fake stars.

* examine the range of star fluxes.  (too many are being generated).  
* check the overlap between the real and random star fluxes

* generate stars with / without known exposure correction (ie, stars
  could be generated with known fluxes or know magnitudes)

* generate galaxies (objects with random values for major, minor, theta)
* generate convolved galaxies

* should not have to supply ZP, Seeing, PA, Scale on command line for all ppSim runs
* DVO.CATDIR path://SIMTEST is not being interpolated?
* -D PSASTRO:DVO.CATDIR is not being interpretted?

* fake stars do not have random magnitudes?
* add generated stars to an output file (pmSource list?)
* make star source an option (random / dvo database)
* add the gain to the metadata

ppSimLoop:

  * load catalogs stars (ra, dec, mag, x, y)
  * generate random stars (ra, dec, mag, x, y)
  * loop over chip,cell,readout
  ** generate output images (signal, variance)
  ** add the bias
  ** add the dark
  ** add the sky signal 
  ** add the stars (modified by the flat, shutter)
  ** add the galaxies (modified by the flat, shutter)
  ** add the poisson noise
  ** add the overscan + readnoise
  ** save the stars
  ** save the galaxies

  ** update the concept info
  ** update the header wcs info
  ** write out the image data

