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.