Get the euler angles.
        The convention is Tait-Bryan (ZY'X'')

        :returns: array containing the euler angles [roll, pitch, yaw]

def euler(self):
        """
        Get the euler angles.
        The convention is Tait-Bryan (ZY'X'')

        :returns: array containing the euler angles [roll, pitch, yaw]
        """
        if self._euler is None:
            if self._q is not None:
                # try to get euler angles from q via DCM
                self._dcm = self._q_to_dcm(self.q)
                self._euler = self._dcm_to_euler(self.dcm)
            elif self._dcm is not None:
                # get euler angles from DCM
                self._euler = self._dcm_to_euler(self.dcm)
        return self._euler