Units missing in output when using pi or netcdf

In output generated by pi_mixin (and in a slightly different way this also holds for netcdf_mixin), the header of each variable has information about the units used to express this variable. Thus far, this information can only be retrieved for variables present in timeseries_import, but other variables in timeseries_export will have a 'unit_unknown' unit. Perhaps it would be possible to infer this information from the dimensions set for the output in the modelica file.