The R factor for merging the reflections that satisfy the
resolution limits established by _diffrn_reflns.d_resolution_high
and _diffrn_reflns.d_resolution_low and the observation limit
established by _diffrn_reflns.observed_criterion.
Rmerge(I) = [sumi(sumj|Ij - |)] / [sumi(sumj)]
Ij = the intensity of the jth observation of reflection i
= 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