MC LandDetector: add constant (0.3) for vz threshold for in_descend flag

Signed-off-by: Silvan Fuhrer <silvan@auterion.com>

src/modules/land_detector/MulticopterLandDetector.h

@@ -87,6 +87,9 @@ private:
 	/** Distance above ground below which entering ground contact state is possible when distance to ground is available. */
 	static constexpr float DIST_FROM_GROUND_THRESHOLD = 1.0f;
 
+	/** Down velocity threshold for setting "in_descend" flag */
+	static constexpr float DESCENT_TRAJECTORY_VZ_THRESHOLD = 0.3f;
+
 	/** Handles for interesting parameters. **/
 	struct {
 		param_t minThrottle;