def add_data(self, data_container):
        name, tags = data_container.name, data_container.tags
        existing_element = self.get_data(name, tags)
        if existing_element is None:
            self._name_index[data_container.name].append(data_container)
        else:
            message = "Element already exists in data container: {}" \
                    .format(existing_element)
            raise ValueError(message)
Returns a string that uniquely identifies a raw result from a particular
dataset/ETHPy150Open impactlab/eemeter/eemeter/meter/base.py/DataCollection.add_data--crossover with--dataset/ETHPy150Open openelections/openelections-core/openelex/us/fl/load.py/LoadResults._key