GROUP

      Assign stars with non-overlapping PSF profiles into distinct groups

      Part of the IDL-DAOPHOT sequence

      GROUP, X, Y, RCRIT, NGROUP

      X - vector, giving X coordinates of a set of stars.
      Y - vector, giving Y coordinates of a set of stars.
           If X and Y are input as integers, then they will be converted to
           floating point
      RCRIT - scalar, giving minimum distance between stars of two
               distinct groups.  Stars less than this distance from
               each other are always in the same group.    Stetson suggests
               setting the critical distance equal to the PSF radius +
               the Fitting radius.

      NGROUP - integer vector, same number of elements as X and Y,
               giving a group number for each star position.  Group
               numbering begins with 0.

      Each position is initially given a unique group number.  The distance
      of each star is computed against every other star.   Those distances
      less than RCRIT are assigned the minimum group number of the set.   A
      check is then made to see if any groups have merged together.

      REM_DUP()

      Written W. Landsman  STX                  April, 1988
      Major revision to properly merge groups together  W. Landsman   Sep 1991
      Work for more than 32767 points    W. Landsman  March 1997
      Converted to IDL V5.0   W. Landsman   September 1997
      Avoid overflow if X and Y are integers      W. Landsman  Feb. 1999

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