Residual factor Rmerge for reflections that satisfy the
               resolution limits established by _reflns.d_resolution_high
               and _reflns.d_resolution_low and the observation limit
               established by _reflns.observed_criterion.
                           sumi(sumj|Fj - <F>|)
               Rmerge(F) = --------------------------
                                sumi(sumj<F>)
               Fj = the amplitude of the jth observation of reflection i
               <F>  = the mean of the amplitudes of all observations of
                      reflection i
               sumi is taken over all reflections
               sumj is taken over all observations of each reflection