
output from ppImage is (in general) a multiple file, multiple
extension set of FITS images:

exp01.chipNN.p2.fits

along with the output object files with the associated astrometry:

exp01.chipNN.p2.cmp

for phase3, output is a single MEF file per exposure with one chips
per extension.  

ISP:
exp01.p2.fits (image data in PHU)
exp01.p2.cmp (astrometry data in PHU, object in table, chip mode)

Megacam:
exp01.p2.fits (one chip per extension)
exp01.p2.cmp (mosaic astrometry in PHU, chip astrometry per extension)

GPC:
exp01.chipNN.p2.fits (one chip per extension)
exp01.chipNN.p2.cmp (astrometry in PHU, chip astrometry per extension)

exp01.p3.cmp (mosaic astrometry in PHU, chip astrometry per extension)


user interface to warp:

warp skycell.fits -image exp01.p2.fits -header exp01.p3.cmp output.fits
 - if -header is not supplied, assume image file contains astrometry
 - skycell astrometry + NAXIS1,2 defines output grid
 - output.fits is created (will be replaced if it exists)
 - allow a subset of the input image to be warped? (specific
 extensions)
 - EXTNAME in input.fits is matched to EXTNAME in header.fits
   - use the WRP / DIS astrometry if available

