Algorithm

  Read parameters

  Read each input image

  If the input images contain a mixture of low and high resolution windows, 
  convert the high-resolution windows to low resolution ones.

  Check that the filters for each file are identical- if not issue a warning.  


  Set output projection (reference coordinate, pixel size) from the
     first input image

  Project corners of each input image into output frame to find the
     bounds of the output image

  Calculate size of output image in pixels from bounds and pixel size

  Create output image

  Loop over input images
     Normalize each one by its exposure time.
     Loop over pixels in output image
        Project coordinate of output pixel into frame of input image
        Sample input pixel, add sample to output image
        Increment each weighting pixel by one if the corresponding input image
        pixel is to be coadded to the corresponding mosaiced image pixel.
        Increment the corresponding exposure image pixel by the exposure time
        for that image.

     End loop
  End loop

  Divide the mosaiced image by the exposure image.
  Multiply the image by the exposure time (default 1000 secs).
  Write output image
  If required, write output exposure map image.