Residual factor R for reflections that satisfy the resolution
limits established by _refine_ls_shell.d_res_high and
_refine_ls_shell.d_res_low and the observation criterion
established by _reflns.observed_criterion.
sum|Fobs - Fcalc|
R = ---------------------
sum|Fobs|
Fobs = the observed structure-factor amplitudes
Fcalc = the calculated structure-factor amplitudes
sum is taken over the specified reflections