AltitudeSmoothVel - Create new altitude flight task that uses the

velocity smoothing library to create a jerk-limited trajectory along the
Z axis. This FlightTask is used when MPC_POS_MODE = 3.

src/modules/mc_pos_control/mc_pos_control_main.cpp

@@ -990,6 +990,10 @@ MulticopterPositionControl::start_flight_task()
 			error =  _flight_tasks.switchTask(FlightTaskIndex::ManualAltitudeSmooth);
 			break;
 
+		case 3:
+			error =  _flight_tasks.switchTask(FlightTaskIndex::ManualAltitudeSmoothVel);
+			break;
+
 		default:
 			error =  _flight_tasks.switchTask(FlightTaskIndex::ManualAltitude);
 			break;