SIGRANGE()

       Selects the most significant data range in an image.

       Selects out the most significant range in the data to be used in
       displaying images.  The histogram of ARRAY is used to select the most
       significant range.      Useful for scaling an image display.

       OUTPUT = SIGRANGE( ARRAY )

       ARRAY    = Array to take most significant range of.

       None.

       The function returns an array where values above and below the
       selected range are set equal to the maximum and minimum of the
       range respectively.

       FRACTION = Fraction of data to consider most significant.
                  Defaults to 0.99
       MISSING  = Value used to flag missing points.  Data points with this
                  value are not considered or changed.

       RANGE    = 2 element vector, giving the range (minimum and maxmimum)
               used

       If the image array contains more than 10,000 points then SIGRANGE()
       uses random indexing of a subset of the points to determine the range
       (for speed).    Thus identical calls to SIGRANGE() might not yield
       identical results (although they should be very close).

       ARRAY must have more than two points.  Fraction must be greater than 0
       and less than 1.
       SIGRANGE was originally part of the SERTS image display package.
       Other routines from this package are available at
       http://sohowww.nascom.nasa.gov/solarsoft/gen/idl/image/
       Note that this version of SIGRANGE does not include the non-standard
       system variables used in the SERTS package.

       Version 1, William Thompson, GSFC, 12 May 1993.
               Incorporated into CDS library.
       Version 2, William Thompson, GSFC, 25 May 1993.
               Changed call to HISTOGRAM to be compatible with OpenVMS/ALPHA
       Version 3, CDP, RAL, Add RANGE keyword.  16-Apr-96
       Version 4, William Thompson, GSFC, 17 April 1996
               Corrected some problems when range is too high.
       Version 5, 13-Jan-1998, William Thompson, GSFC
               Use random numbers to improve statistics when only using a
               fraction of the array.
       Version 6, 06-Mar-1998, William Thompson, GSFC
               Change default to 0.99

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