Set the DCM
        :param dcm: Matrix3

def dcm(self, dcm):
        """
        Set the DCM
        :param dcm: Matrix3

        """
        assert(isinstance(dcm, Matrix3))
        self._dcm = dcm.copy()

        # mark other representations as outdated, will get generated on next
        # read
        self._q = None
        self._euler = None