The highest resolution for the interplanar spacing in the
               reflection data in this shell. This is the smallest d value.
               Note that the resolution limits of shells in the items
               _phasing_MIR_shell.d_res_high and _phasing_MIR_shell.d_res_low
               are independent of the resolution limits of shells in the items
               _reflns_shell.d_res_high and _reflns_shell.d_res_low.