mtz: if a file has no datasets, automatically add HKL_base
mtz: if a file has no datasets, automatically add HKL_base

"From CCP4 5.0, there should always be a base dataset, called HKL_base
and with dataset ID of zero. This dataset will be added automatically by
the library, if it doesn't already exist."

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