FITS_TEST_CHECKSUM()

    Verify the values of the CHECKSUM and DATASUM keywords in a FITS header

     Follows the 23 May 2002 version of the FITS checksum proposal at
     http://heasarc.gsfc.nasa.gov/docs/heasarc/fits/checksum.html

    result = FITS_TEST_CHECKSUM(HDR, [ DATA, ERRMSG=, /FROM_IEEE ])

    HDR - FITS header (vector string)

    DATA - data array associated with the FITS header.   If not supplied, or
           set to a scalar, then there is assumed to be no data array
           associated with the FITS header.

     An integer -1, 0 or 1 indicating the following conditions:
           1 - CHECKSUM (and DATASUM) keywords are present with correct values
           0 - CHECKSUM keyword is not present
          -1 - CHECKSUM or DATASUM keyword does not have the correct value
               indicating possible data corruption.

    /FROM_IEEE - If this keyword is set, then the input is assumed to be in
             big endian format (e.g. an untranslated FITS array).    This
             keyword only has an effect on little endian machines (e.g.
             a Linux box).

     ERRMSG - will contain a scalar string giving the error condition.   If
              RESULT = 1 then ERRMSG will be an empty string.   If this
              output keyword is not supplied, then the error message will be
              printed at the terminal.

     The header and data must be *exactly* as originally written in the FITS
     file.  By default, some FITS readers may alter keyword values (e.g.
     BSCALE) or append information (e.g. HISTORY or an inherited primary
     header) and this will alter the checksum value.

    CHECKSUM32, FITS_ASCII_ENCODE(), SXPAR()

     Verify the CHECKSUM keywords in the primary header/data unit of a FITS
     file 'test.fits'
     FITS_READ,'test.fits',data,hdr,/no_PDU,/NoSCALE
     print,FITS_TEST_CHECKSUM(hdr,data)
     Note the use of the /No_PDU and /NoSCALE keywords to avoid any alteration
     of the FITS header

     W. Landsman  SSAI               December 2002
     Return quietly if CHECKSUM keywords not found W. Landsman May 2003
     Add /NOSAVE to CHECKSUM32 calls when possible W. Landsman Sep 2004

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