AP_AHRS: move groundspeed vector up to AP_AHRS_DCM

The other estimators all have their own implementation of this

libraries/AP_AHRS/AP_AHRS_Backend.cpp

@@ -77,7 +77,7 @@ void AP_AHRS::update_orientation()
 }
 
 // return a ground speed estimate in m/s
-Vector2f AP_AHRS_Backend::groundspeed_vector(void)
+Vector2f AP_AHRS_DCM::groundspeed_vector(void)
 {
     // Generate estimate of ground speed vector using air data system
     Vector2f gndVelADS;

libraries/AP_AHRS/AP_AHRS_Backend.h

@@ -207,7 +207,7 @@ public:
     }
 
     // return a ground vector estimate in meters/second, in North/East order
-    virtual Vector2f groundspeed_vector(void);
+    virtual Vector2f groundspeed_vector(void) = 0;
 
     // return a ground velocity in meters/second, North/East/Down
     // order. This will only be accurate if have_inertial_nav() is
@@ -465,12 +465,6 @@ protected:
     Vector3f        _accel_ef[INS_MAX_INSTANCES];
     Vector3f        _accel_ef_blended;
 
-    // Declare filter states for HPF and LPF used by complementary
-    // filter in AP_AHRS::groundspeed_vector
-    Vector2f _lp; // ground vector low-pass filter
-    Vector2f _hp; // ground vector high-pass filter
-    Vector2f _lastGndVelADS; // previous HPF input
-
     // helper trig variables
     float _cos_roll{1.0f};
     float _cos_pitch{1.0f};

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -101,6 +101,9 @@ public:
         return true;
     }
 
+    // return a ground vector estimate in meters/second, in North/East order
+    Vector2f groundspeed_vector() override;
+
     bool            use_compass() override;
 
     // return the quaternion defining the rotation from NED to XYZ (body) axes
@@ -232,4 +235,10 @@ private:
 
     // last origin we returned, for DCM fallback from EKF
     Location last_origin;
+
+    // Declare filter states for HPF and LPF used by complementary
+    // filter in AP_AHRS::groundspeed_vector
+    Vector2f _lp; // ground vector low-pass filter
+    Vector2f _hp; // ground vector high-pass filter
+    Vector2f _lastGndVelADS; // previous HPF input
 };