Converts Matrix3 in an array
        :param m: Matrix3
        :returns: 3x3 array

def _matrix3_to_dcm_array(self, m):
        """
        Converts Matrix3 in an array
        :param m: Matrix3
        :returns: 3x3 array
        """
        assert(isinstance(m, Matrix3))
        return np.array([[m.a.x, m.a.y, m.a.z],
                         [m.b.x, m.b.y, m.b.z],
                         [m.c.x, m.c.y, m.c.z]])