For each reflection class, the weighted residual factor for all
               reflections included in the refinement. The reflections also
               satisfy the resolution limits established by
               _refine_ls_class.d_res_high and _refine_ls_class.d_res_low.
               See also the _refine_ls_class.R_factor_ definitions.
                    ( sum w [ Y(obs) - Y(calc) ]2  )1/2
               wR = ( ------------------------------ )
                    (         sum w Y(obs)2       )
               Y(obs)  = the observed amplitude specified by
                         _refine.ls_structure_factor_coef
               Y(calc) = the calculated amplitude specified by
                         _refine.ls_structure_factor_coef
               w       = the least-squares weight
               and the sum is taken over the reflections of this class.