HREBIN

    Expand or contract a FITS image using (F)REBIN and update the header

    If the output size is an exact multiple of the input size then REBIN is
    used, else FREBIN is used.     User can either overwrite the input array,
    or write to new variables.

    HREBIN, oldhd        ;Special calling sequence to just update header
    HREBIN, oldim, oldhd, [ newim, newhd, newx, newy, OUTSIZE = ,/SAMPLE,
                            ERRMSG =  ]

    OLDIM - the original image array
    OLDHD - the original image FITS header, string array

    NEWX - size of the new image in the X direction, integer scalar
    NEWY - size of the new image in the Y direction, integer scalar
            HREBIN will prompt for NEWX and NEWY if not supplied

    NEWIM - the image after expansion or contraction with REBIN
    NEWHD - header for newim containing updated astrometry info
            If output parameters are not supplied, the program will modify
            the input parameters OLDIM and OLDHD to contain the new array and
            updated header.

    /SAMPLE - Expansion or contraction is done using REBIN which uses
              bilinear interpolation when magnifying and boxaveraging when
              minifying.   If the SAMPLE keyword is supplied and non-zero,
              then nearest neighbor sampling is used in both cases.   Keyword
              has no effect when output size is not a multiple of input size.
    OUTSIZE - Two element integer vector which can be used instead of the
             NEWX and NEWY parameters to specify the output image dimensions
    ALT - Single character 'A' through 'Z' or ' ' specifying which astrometry
          system to modify in the FITS header.    The default is to use the
          primary astrometry of ALT = ' '.    See Greisen and Calabretta (2002)
          for information about alternate astrometry keywords.

       ERRMSG - If this keyword is supplied, then any error mesasges will be
               returned to the user in this parameter rather than depending on
               on the MESSAGE routine in IDL.   If no errors are encountered
               then a null string is returned.

     The parameters BSCALE, NAXIS1, NAXIS2, CRPIX1, and CRPIX2 and the CD
     (or CDELT) parameters are updated for the new FITS header.

     Compress a 2048 x 2048 image array IM, with FITS header HDR, to a
     724 x 724 array.   Overwrite the input variables with the compressed
     image and header.
     IDL> hrebin, im, hdr, OUT = [724, 724]

     CHECK_FITS, EXTAST, FREBIN, GSSS_STDAST, STRN(), SXPAR(), SXADDHIST,
     SXADDPAR, ZPARCHECK

     Written, December 1990  W. Landsman, ST System Corp.
     Update CD1_1 keywords   W. Landsman   November 1992
     Check for a GSSS header   W. Landsman  June 1994
     Update BSCALE even if no astrometry present   W. Landsman  May 1997
     Converted to IDL V5.0   W. Landsman   September 1997
     Use FREBIN to accept sizes that are not a integer multiple of the original
         size    W. Landsman     August 1998
     Correct for "edge" effects when expanding with REBIN W. Landsman Apr. 1999
     Fixed initialization of header only call broken in Apr 98 change May. 1999
     Remove reference to obsolete !ERR  W. Landsman   February 2000
     Use double precision formatting for CD matrix W. Landsman April 2000
     Recognize PC00n00m astrometry format   W. Landsman   December 2001
     Correct astrometry for integral contraction W. Landsman  April 2002
     Fix output astrometry for non-equal plate scales for PC matrix or
     CROTA2 keyword, added ALT keyword.   W. Landsman May 2005

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