do a full 3D accel calibration

def cmd_accelcal(self, args):
        '''do a full 3D accel calibration'''
        mav = self.master
        # ack the APM to begin 3D calibration of accelerometers
        mav.mav.command_long_send(mav.target_system, mav.target_component,
                                  mavutil.mavlink.MAV_CMD_PREFLIGHT_CALIBRATION, 0,
                                  0, 0, 0, 0, 1, 0, 0)
        self.accelcal_count = 0
        self.accelcal_wait_enter = False