FITS_ASCII_ENCODE()

    Encode an unsigned longword as an ASCII string to insert 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_ASCII_ENCODE( sum32)

     sum32 - 32bit *unsigned longword* (e.g. as returned by CHECKSUM32)

     A 16 character scalar string suitable for the CHECKSUM keyword

      A FITS header/data unit has a checksum of 868229149.  Encode the
      complement of this value (3426738146) into an ASCII string
      IDL> print,FITS_ASCII_ENCODE(3426738146U)
           ===> "hcHjjc9ghcEghc9g"

      The 32bit value is interpreted as a sequence of 4 unsigned 8 bit
      integers, and divided by 4.    Add an offset of 48b (ASCII '0').
      Remove non-alphanumeric ASCII characters (byte values 58-64 and 91-96)
      by simultaneously incrementing and decrementing the values in pairs.
      Cyclicly shift the string one place to the right.

     Written  W. Landsman  SSAI              December 2002

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