Iterate through semesters, counties and municipalities.

def _fetch_dimensions(self, dataset):
        """ Iterate through semesters, counties and municipalities.
        """
        yield Dimension(u"school")
        yield Dimension(u"year",
                        datatype="year")
        yield Dimension(u"semester",
                        datatype="academic_term",
                        dialect="swedish")  # HT/VT
        yield Dimension(u"municipality",
                        datatype="year",
                        domain="sweden/municipalities")