AP_SpdHgtControl: added get_max_sinkrate()

libraries/AP_SpdHgtControl/AP_SpdHgtControl.h

@@ -50,6 +50,9 @@ public:
 	// return maximum climb rate
 	virtual float get_max_climbrate(void) const = 0;
 
+    // return maximum sink rate (+ve number)
+    virtual float get_max_sinkrate(void) const = 0;
+
     // added to let SoaringController reset pitch integrator to zero
     virtual void reset_pitch_I(void) = 0;