MC auto: add parameter to set the trajectory slow-down

In some cases e.g.: (VTOL in wind) a good tracking cannot be
achieved. This condition then needs to be relaxed, otherwise the
drone cannot land properly.

src/modules/mc_pos_control/mc_pos_control_params.c

@@ -281,6 +281,24 @@ PARAM_DEFINE_FLOAT(MPC_XY_CRUISE, 5.0f);
  */
 PARAM_DEFINE_FLOAT(MPC_XY_TRAJ_P, 0.5f);
 
+/**
+ * Maximum horizontal error allowed by the trajectory generator
+ *
+ * The integration speed of the trajectory setpoint is linearly
+ * reduced with the horizontal position tracking error. When the
+ * error is above this parameter, the integration of the
+ * trajectory is stopped to wait for the drone.
+ *
+ * This value can be adjusted depending on the tracking
+ * capabilities of the vehicle.
+ *
+ * @min 0.1
+ * @max 10.0
+ * @decimal 1
+ * @group Multicopter Position Control
+ */
+PARAM_DEFINE_FLOAT(MPC_XY_ERR_MAX, 2.0f);
+
 /**
  * Maximum horizontal velocity setpoint for manual controlled mode
  * If velocity setpoint larger than MPC_XY_VEL_MAX is set, then