restore _fCut that was lost in AP_Param development

libraries/PID/PID.cpp

@@ -30,7 +30,7 @@ PID::get_pid(int32_t error, uint16_t dt, float scaler)
 
 		// discrete low pass filter, cuts out the
 		// high frequency noise that can drive the controller crazy
-		float RC = 1/(2*M_PI*20);
+		float RC = 1/(2*M_PI*_fCut);
 		derivative = _last_derivative +
 		        (delta_time / (RC + delta_time)) * (derivative - _last_derivative);
 

libraries/PID/PID.h

@@ -82,6 +82,13 @@ private:
 	float				_integrator;		///< integrator value
 	int32_t				_last_error;		///< last error for derivative
 	float				_last_derivative; 	///< last derivative for low-pass filter
+
+	/// Low pass filter cut frequency for derivative calculation.
+	///
+	/// 20 Hz becasue anything over that is probably noise, see
+	/// http://en.wikipedia.org/wiki/Low-pass_filter.
+	///
+	static const uint8_t _fCut = 20;
 };
 
 #endif