PID: added get_pid_4500()

this is a version of get_pid() that returns an int16_t constrained to
-4500 to 4500. This will prevent overflow errors for large PID gains
in ArduPlane and Rover

libraries/PID/PID.cpp

@@ -88,6 +88,12 @@ float PID::get_pid(float error, float scaler)
     return output;
 }
 
+int16_t PID::get_pid_4500(float error, float scaler)
+{
+	float v = get_pid(error, scaler);
+	return constrain(v, -4500, 4500);
+}
+
 void
 PID::reset_I()
 {

libraries/PID/PID.h

@@ -42,6 +42,9 @@ public:
     ///
     float        get_pid(float error, float scaler = 1.0);
 
+	// get_pid() constrained to +/- 4500
+    int16_t     get_pid_4500(float error, float scaler = 1.0);
+
     /// Reset the PID integrator
     ///
     void        reset_I();