There are multiple units components in the DSD of pivoted multi-measure cubes

[nimshi89]

This ticket is related to #703. There has been an error where the unit's component is duplicated in pivoted multi-measure cubes. This causes the problem that we cannot uniquely (and correctly) link the units component to the relevant column.

In dsdtordfmodelhelper.py the modification has been made (by Rob) to avoid the unit column duplication.

   def _get_qb_units_column_specification(
        self, column_name_uri_safe: str
    ) -> List[rdf.qb.AttributeComponentSpecification]:
        if self._units_component_already_defined:
            # Don't define a second units component, the first one will work just fine.
            return []

        self._units_component_already_defined = True

This method ensures that if a units column has already been defined, then another column cannot be recorded.