Residual factor Rmerge for reflections that satisfy the
resolution limits established by _reflns_shell.d_res_high and
_reflns_shell.d_res_low and the observation criterion
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