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.
                   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