The value of Rmerge(I) for reflections classified as 'observed'
(see _reflns.observed_criterion) in a given shell.
sum~i~(sum~j~|I~j~ - |)
Rmerge(I) = --------------------------
sum~i~(sum~j~)
I~j~ = the intensity of the jth observation of reflection i
= the mean of the intensities of all observations of
reflection i
sum~i~ is taken over all reflections
sum~j~ is taken over all observations of each reflection