Parameters

 Keyword  Posn   Type        Description
 -------  ----   ----        -----------
 DATA       1    UNIV        Dataset containing data values
           GLOBAL.FITDAT (Default fit data)
 MODEL      2    UNIV        Dataset containing model values
           GLOBAL.FITMOD (Default fit model)
 POISSON    -    LOGICAL     Poisson data errors to be assumed?
        Default = YES