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