Set the DCM
        :param dcm: 3x3 array

def dcm(self, dcm):
        """
        Set the DCM
        :param dcm: 3x3 array

        """
        assert(len(dcm) == 3)
        for sub in dcm:
            assert(len(sub) == 3)

        self._dcm = np.array(dcm)

        # mark other representations as outdated, will get generated on next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