AP_AHRS: add the get_variances method found in AP_AHRS_NavEKF to AP_AHRS

libraries/AP_AHRS/AP_AHRS.h

@@ -442,6 +442,14 @@ public:
         return false;
     }
     
+    // get_variances - provides the innovations normalised using the innovation variance where a value of 0
+    // indicates prefect consistency between the measurement and the EKF solution and a value of of 1 is the maximum
+    // inconsistency that will be accpeted by the filter
+    // boolean false is returned if variances are not available
+    virtual bool get_variances(float &velVar, float &posVar, float &hgtVar, Vector3f &magVar, float &tasVar, Vector2f &offset) const {
+        return false;
+    }
+    
     // time that the AHRS has been up
     virtual uint32_t uptime_ms(void) const = 0;
 

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -214,7 +214,7 @@ public:
     // indicates prefect consistency between the measurement and the EKF solution and a value of of 1 is the maximum
     // inconsistency that will be accpeted by the filter
     // boolean false is returned if variances are not available
-    bool get_variances(float &velVar, float &posVar, float &hgtVar, Vector3f &magVar, float &tasVar, Vector2f &offset) const;
+    bool get_variances(float &velVar, float &posVar, float &hgtVar, Vector3f &magVar, float &tasVar, Vector2f &offset) const override;
 
     // returns the expected NED magnetic field
     bool get_mag_field_NED(Vector3f& ret) const;