Looks up a local actor by its location relative to this actor.

def lookup_cell(self, uri):
        """Looks up a local actor by its location relative to this actor."""
        steps = uri.steps
        if steps[0] == '':
            found = self.root
            steps.popleft()
        else:
            found = self
        for step in steps:
            assert step != ''
            found = found.get_child(step)
            if not found:
                break
            found = found._cell
        return found