The value of Rmerge(F) for significantly intense reflections
(see _reflns.threshold_expression) in a given shell.
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.