Residual factor Rmerge for all reflections that satisfy the
resolution limits established by _reflns.d_resolution_high
and _reflns.d_resolution_low.
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