cif2mtz: in unmerged mtz output a single dataset (batch-set)
cif2mtz: in unmerged mtz output a single dataset (batch-set)

previously, it was incorrect for multiple diffrn_id's - dataset id in
batch header was not matching any dataset in normal header

project-gemmi/gemmi,project-gemmi/gemmi,project-gemmi/gemmi,project-gemmi/gemmi