AP_NavEKF2: pass quaternion by const reference

libraries/AP_NavEKF2/AP_NavEKF2_core.cpp

@@ -1379,7 +1379,7 @@ void NavEKF2_core::StoreQuatReset()
 }
 
 // Rotate the stored output quaternion history through a quaternion rotation
-void NavEKF2_core::StoreQuatRotate(Quaternion deltaQuat)
+void NavEKF2_core::StoreQuatRotate(const Quaternion &deltaQuat)
 {
     outputDataNew.quat = outputDataNew.quat*deltaQuat;
     // write current measurement to entire table

libraries/AP_NavEKF2/AP_NavEKF2_core.h

@@ -527,7 +527,7 @@ private:
     void StoreQuatReset(void);
 
     // Rotate the stored output quaternion history through a quaternion rotation
-    void StoreQuatRotate(Quaternion deltaQuat);
+    void StoreQuatRotate(const Quaternion &deltaQuat);
 
     // store altimeter data
     void StoreBaro();