FITS_ADD_CHECKSUM

    Add or update 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

     FITS_ADD_CHECKSUM, Hdr, [ Data, /No_TIMESTAMP, /FROM_IEEE ]

     Hdr - FITS header (string array), it will be updated with new
           (or modfied) CHECKSUM and DATASUM keywords

     Data - data array associated with the FITS header.   If not supplied, or
           set to a scalar, then the program checks whether there is a
           DATASUM keyword already in the FITS header containing the 32bit
           checksum for the data.   if there is no such keyword then ther
           assumed to be no data array
           associated with the FITS header.

    /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).
    /No_TIMESTAMP - If set, then a time stamp is not included in the comment
             field of the CHECKSUM and DATASUM keywords.   Unless the
             /No_TIMESTAMP keyword is set, repeated calls to FITS_ADD_CHECKSUM
             with the same header and data will yield different values of
             CHECKSUM (as the date stamp always changes).   However, use of the
             date stamp is recommended in the checksum proposal.

     CHECKSUM32, FITS_ASCII_ENCODE(), GET_DATE, SXADDPAR, SXPAR()

     W. Landsman    SSAI    December 2002

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