XMEDSKY

       Subtract sky from an image as a 1-D function of X

       This procedure is designed to remove the sky from slitless spectra.
       The sky is assumed to vary with wavelength (along a row) but not with
       position (along a column).    The sky is computed as the
       column-by-column median of pixels within 3 sigma of the image global
       median.   This procedure is called by the cosmic ray rejection routine
       CR_REJECT

       XMEDSKY, Image, Bkg, [ CLIP=[x0, x1, y0, y1], NSIG= ]

       Image:  Input image for which sky vector is to be computed.

       CLIP:   [x0, x1, y0, y1]: region of image to be used for all
               statistical computations.    Default is to use the entire
               image.   For STIS 1024 x 512 slitless spectra, the suggested
               value is CLIP = [32,1023,12,499]
       NSIG:   Positive scalar giving the number of sigma a pixel must be above
               the global median to be reject.   Default is 3 sigma.

       Bkg:    Vector of sky values.
;

       Written by:     R. S. Hill, Hughes STX, 20 Oct. 1997
       Converted to V5.0, use STDDEV()   W. Landsman   June 1998
       Check for valid WHERE, added NSIG keyword  W. Landsman   December 2000
       Assume since V5.1 so always use STDDEV  W. Landsman Feb 2004

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