Parse the root XML element as an aggregate.

def parse_at_root(
            self,
            root,  # type: ET.Element
            state  # type: _ProcessorState
    ):
        # type: (...) -> Any
        """Parse the root XML element as an aggregate."""
        parsed_dict = self._dictionary.parse_at_root(root, state)
        return self._converter.from_dict(parsed_dict)