The value of Rmerge(I) for reflections classified as 'observed'
               (see _reflns.observed_criterion) 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