Support gliding setpoints for autonomous soaring (#14643)

* Enable offboard gliding position setpoints

Adds a capability to make the vehicle glide when given position setpoints.

Gliding setpoints are enabled by setting POSITION_TARGET_TYPEMASK_Z_IGNORE, POSITION_TARGET_TYPEMASK_VZ_IGNORE,  POSITION_TARGET_TYPEMASK_AZ_IGNORE on the typemask of position setpoints.
Include zero cruising throttle
Invalid setpoints with nan

* Handle nans for curise_throttle

src/modules/navigator/mission.cpp

@@ -1912,6 +1912,7 @@ bool Mission::position_setpoint_equal(const position_setpoint_s *p1, const posit
 		(p1->acceleration_is_force == p2->acceleration_is_force) &&
 		(fabsf(p1->acceptance_radius - p2->acceptance_radius) < FLT_EPSILON) &&
 		(fabsf(p1->cruising_speed - p2->cruising_speed) < FLT_EPSILON) &&
-		(fabsf(p1->cruising_throttle - p2->cruising_throttle) < FLT_EPSILON));
+		((fabsf(p1->cruising_throttle - p2->cruising_throttle) < FLT_EPSILON) || (!PX4_ISFINITE(p1->cruising_throttle)
+				&& !PX4_ISFINITE(p2->cruising_throttle))));
 
 }

src/modules/navigator/navigator.h

@@ -240,7 +240,7 @@ public:
 	/**
 	 * Set the target throttle
 	 */
-	void		set_cruising_throttle(float throttle = -1.0f) { _mission_throttle = throttle; }
+	void		set_cruising_throttle(float throttle = NAN) { _mission_throttle = throttle; }
 
 	/**
 	 * Get the acceptance radius given the mission item preset radius
@@ -393,7 +393,7 @@ private:
 
 	float _mission_cruising_speed_mc{-1.0f};
 	float _mission_cruising_speed_fw{-1.0f};
-	float _mission_throttle{-1.0f};
+	float _mission_throttle{NAN};
 
 	// update subscriptions
 	void		params_update();

src/modules/navigator/navigator_main.cpp

@@ -907,7 +907,7 @@ Navigator::get_cruising_throttle()
 		return _mission_throttle;
 
 	} else {
-		return -1.0f;
+		return NAN;
 	}
 }