The value of Rmerge(I) for significantly intense reflections
               (see _reflns.threshold_expression) in a given shell.
                           sumi ( sumj | Ij - <I> | )
               Rmerge(I) = --------------------------------
                               sumi ( sumj <I> )
               Ij  = the intensity of the jth observation of reflection i
               <I> = the mean of the intensities of all observations of
                      reflection i
               sumi is taken over all reflections
               sumj is taken over all observations of each reflection.