PURPOSE:   (one line only)
  Stack a set of images on a given ROI warped to a master image
  This tool is designed to build a reference image for a given image of
    the sky.  The reference image is intended to be subtracted by a later
    tool to remove the fixed stellar background.  The template is built
    by resampling the input reference images to precisely match the
    master image, which includes its scale, orientation and any optical
    distortions.  The input list of reference images should be a reasonble
    list of images to be considered but not all of them are required to be
    useful for this purpose (ie., no overlap).  However, it is up to the
    calling program to not propose reference images that contain the object
    of interest on or near the location of that object in the master image.

  The stack is built from those reference images that have overlap with
    the ROI on the master image.  The software works to use only the best
    data from the reference image list to make the template.

  When complete, four files are saved.  1) a log file containg information
    about the stacking process, 2) the stacked image in FITS format, 3) the
    cube from which the stack was built, also in FITS format, and 4) a screen
    shot of the template image saved in .png format

  Construction of templates works best where there are lots of images to
    work with but this tool strives to make do if the number of files is
    small.  With 5 or more images, it will do a robust average of the cube.
    With 4 or less a straight median is used.  If there is only one relevant
    file you get that one file, resampled to match but it will still have
    all image blemishes, like cosmic ray strikes.

  CCD Data Processing
  fnim    - File name of the master image that the stack will be built to match
  fnref   - List of files that may be useful for building a stack
  FWHM    - Vector of fwhm, in pixels for each image in fnref.  If provided,
               this can speed up the program significantly.  The default
               is to determine this on the images internally.
  GAIN    - Gain of detector, default is 1e-/DN
  KEYFILE - Header keyword list (see
  OUTPATH - Directory where output files are to be written to.  Default is
               to write to the current directory.
  PATH    - Directory where input images (master and reference) can be
               found.  You can put paths on the inputs in which case use
               the default of '' for this keyword.
  ROI     - Region of interest from master image that stack will be built
               to.  The default is to do it for the entire image.  Provide
               a 4-element vector [i0,j0,i1,j1] (lower left hand corner and
               upper right hand corner of region) to use this option.
  VERBOSE - Flag, if set will turn on a lot of printed output to see what
              the program is doing.
  MINAREA - minimum fraction of area overlap allowed to keep a template
              image.  Default = 0.65
  error - Flag, set to 1 in case of error.  The
  This tool depends on the presence of WCS information in the image headers
    and expects to find my format information in there (see
  2016/04/12, Written by Marc W. Buie, Southwest Research Institute
  2016/04/21, MWB, slight fixes to logic for output file names when
               input files contain their paths.
  2016/09/29, MWB, added MINAREA keyword