AP_AHRS: added get_DCM_rotation_body_to_ned()

this specifically asks for the DCM attitude, which will be used for
the comass calibration to help determine compass orientation

libraries/AP_AHRS/AP_AHRS.h

@@ -263,6 +263,9 @@ public:
     const Matrix3f& get_rotation_autopilot_body_to_vehicle_body(void) const { return _rotation_autopilot_body_to_vehicle_body; }
     const Matrix3f& get_rotation_vehicle_body_to_autopilot_body(void) const { return _rotation_vehicle_body_to_autopilot_body; }
 
+    // get rotation matrix specifically from DCM backend (used for compass calibrator)
+    virtual const Matrix3f &get_DCM_rotation_body_to_ned(void) const = 0;
+    
     // get our current position estimate. Return true if a position is available,
     // otherwise false. This call fills in lat, lng and alt
     virtual bool get_position(struct Location &loc) const = 0;

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -53,6 +53,9 @@ public:
         return _body_dcm_matrix;
     }
 
+    // get rotation matrix specifically from DCM backend (used for compass calibrator)
+    const Matrix3f &get_DCM_rotation_body_to_ned(void) const override { return _body_dcm_matrix; }
+    
     // return the current drift correction integrator value
     const Vector3f &get_gyro_drift() const override {
         return _omega_I;