NAME:
    digit
 PURPOSE: (one line)
    Digitize from a displayed gif file.
 DESCRIPTION:
 CATEGORY:
    Image display
 CALLING SEQUENCE:
    digit, image, outfile
 INPUTS:
    image   : Image to be displayed.  May be a string scalar containing the
              name of an image file to be loaded and displayed, or an image
              array to be displayed.  File is read with READ_IMAGE.
    outfile : File into which collected points are saved.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
    NATURAL    = If set, the natural screen coordinates are used.  No
                 control points are solicited and no coordinate
                 transformations are computed.
    NOSCALE    = If set, the image is displayed without being scaled.
    ROTATE     = Rotation angle (in degrees) for the gif image.  Allowed
                 values are 90, 180, and 270 counterclockwise.
 OUTPUTS:
 KEYWORD OUTPUT PARAMETERS:
    XVEC, YVEC = If both of these keywords are present, the collected
                 coordinates are returned as vectors to the variables
                 specified in the keywords.  The collected coordinates are
                 always written to the output file.
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
    Unless the 'NATURAL' keyword is specified, four control points and four
 associated values are requested in order to define the x and y axes and
 scale factors.
    The first two points are the x-axis control points (and their values) and
 the second two are the y-axis control points (and their values).
    Control vectors are computed from the requested control points and
 coordinate transformations are computed using simple vector algebra.
 MODIFICATION HISTORY:
  Written by Doug Loucks, Lowell Observatory, April, 1993.
  94/10/04 - MWB - moved cr to front of print statement (v3.6 bug workaround)
  2000/01/31, MWB, fixed bug that was introduced by some IDL version that
                      changed the way CURSOR worked.
  2009/01/13, MWB, changed to use READ_IMAGE to read external file.