IMU: added get_gyro_drift_rate() interface

this just calls down to the InertialSensor layer

libraries/AP_IMU/AP_IMU_INS.cpp

@@ -250,3 +250,10 @@ AP_IMU_INS::update(void)
 bool AP_IMU_INS::new_data_available(void) {
     return _ins->new_data_available();
 }
+
+/// return the maximum gyro drift rate in radians/s/s. This
+/// depends on what gyro chips are being used
+float AP_IMU_INS::get_gyro_drift_rate(void)
+{
+    return _ins->get_gyro_drift_rate();
+}

libraries/AP_IMU/AP_IMU_INS.h

@@ -64,6 +64,7 @@ public:
 	virtual void		ax(const float v)		{ _sensor_cal[3] = v; }
 	virtual void		ay(const float v)		{ _sensor_cal[4] = v; }
 	virtual void		az(const float v)		{ _sensor_cal[5] = v; }
+    virtual float       get_gyro_drift_rate(void);
 
 private:
     AP_InertialSensor   *_ins;          ///< INS provides an axis and unit correct sensor source.

libraries/AP_IMU/AP_IMU_Shim.h

@@ -67,6 +67,8 @@ public:
 	// dummy save method
 	void		save(void) { }
 
+	float get_gyro_drift_rate(void) { return 0; }
+
 private:
 	/// set true when new data is delivered
 	bool		_updated;

libraries/AP_IMU/IMU.cpp

@@ -41,3 +41,4 @@ float	IMU::az(void) { return 0.0; }
 void	IMU::ax(const float v) { }
 void	IMU::ay(const float v) { }
 void	IMU::az(const float v) { }
+float   IMU::get_gyro_drift_rate(void) { return 0; }

libraries/AP_IMU/IMU.h

@@ -84,6 +84,10 @@ public:
 	///
 	float			get_delta_time(void) { return _sample_time * 1.0e-6; }
 
+	/// return the maximum gyro drift rate in radians/s/s. This
+	/// depends on what gyro chips are being used
+	virtual float	get_gyro_drift_rate(void);
+
 	/// A count of bad sensor readings
 	///
 	/// @todo This should be renamed, as there's no guarantee that sensors