The highest value for the interplanar spacings for the
reflection data in this shell. This is called the lowest
resolution. 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.