Add an intervention to vectorPop section.
        intervention is either ElementTree or xml snippet

def add(self, intervention, name=None):
        """
        Add an intervention to vectorPop section.
        intervention is either ElementTree or xml snippet
        """
        if self.et is None:
            return

        assert isinstance(intervention, six.string_types)
        et = ElementTree.fromstring(intervention)
        vector_pop = VectorPopIntervention(et)

        assert isinstance(vector_pop.name, six.string_types)

        if name is not None:
            assert isinstance(name, six.string_types)
            et.attrib["name"] = name

        index = len(self.et.findall("intervention"))
        self.et.insert(index, et)