APM_Control: added decay_I() function

used by VTOL planes to decay integrator on fixed wing components when
at very low airspeed

libraries/APM_Control/AP_PitchController.h

@@ -26,6 +26,14 @@ public:
 
 	void reset_I();
 
+    /*
+      reduce the integrator, used when we have a low scale factor in a quadplane hover
+    */
+    void decay_I() {
+        // this reduces integrator by 95% over 2s
+        _pid_info.I *= 0.995f;
+    }
+    
     void autotune_start(void) { autotune.start(); }
     void autotune_restore(void) { autotune.stop(); }
 

libraries/APM_Control/AP_RollController.h

@@ -26,6 +26,14 @@ public:
 
 	void reset_I();
 
+    /*
+      reduce the integrator, used when we have a low scale factor in a quadplane hover
+    */
+    void decay_I() {
+        // this reduces integrator by 95% over 2s
+        _pid_info.I *= 0.995f;
+    }
+    
     void autotune_start(void) { autotune.start(); }
     void autotune_restore(void) { autotune.stop(); }
 

libraries/APM_Control/AP_YawController.h

@@ -26,6 +26,14 @@ public:
 
 	void reset_I();
 
+    /*
+      reduce the integrator, used when we have a low scale factor in a quadplane hover
+    */
+    void decay_I() {
+        // this reduces integrator by 95% over 2s
+        _pid_info.I *= 0.995f;
+    }
+    
 	const AP_Logger::PID_Info& get_pid_info(void) const {return _pid_info; }
 
 	static const struct AP_Param::GroupInfo var_info[];