
need to update the following functions:

ppImage/src/ppImageDetrendReadout.c:        pmReadoutGenerateVariance(input, true);
ppSim/src/ppSimLoop.c:                    if (!pmReadoutGenerateVariance(readout, true)) {
ppSim/src/ppSimMergeReadouts.c:      if (!pmReadoutGenerateVariance(inReadout, true)) {
psphot/src/psphotMaskReadout.c:        if (!pmReadoutGenerateVariance(readout, true)) {

plus: 
  psModules/src/camera/pmFPAMaskWeight.c:bool pmReadoutSetVariance(pmReadout *readout, bool poisson)
  psModules/src/camera/pmFPAMaskWeight.c:bool pmReadoutGenerateVariance(pmReadout *readout, bool poisson)
  psModules/src/camera/pmFPAMaskWeight.c:bool pmReadoutGenerateMaskVariance(pmReadout *readout, psImageMaskType satMask, psImageMaskType badMask, bool poisson)


i'm looking into defining the readnoise for ppImage at a finer level
of resolution than by cell: this will let us set thresholds which are
prevent excessive magic false positives in the regions with bad
correlated noise, but still allow detection of streaks and faint
sources in the regions on the same cells with low correlated noise.

my proposal is as follows:

* define the readnoise map using a superpixel map image for each cell,
equivalent to the background model, with data saved in a FITS file
collection parallel to the other camera images -- for GPC1, that would
mean an extension per cell and a file per chip.

* save the readnoise map as a detrend type, and load it with the
detrend loading mechanisms.

* pass the information to pmReadoutSetVariance with a pointer to the
psImageMap structure, which defines the super-pixel image and the
information about overlaps needed to line up the fine and rough
resolution images.

* trigger the selection based on a concept CELL.READNOISE.MAP (a bool).

I think the mechanism will be useful even if it turns out that JT can
correct the images -- there may still be sub-cell variations in the
effective readnoise level.

* how to read/write the map:
  * ppImage output file?
  
