AP_InertialNav: rename get_velocity_xy to get_speed_xy

A velocity is a vector - since we return just a float, this is a speed

libraries/AP_InertialNav/AP_InertialNav.h

@@ -86,11 +86,11 @@ public:
     virtual const Vector3f&    get_velocity() const = 0;
 
     /**
-     * get_velocity_xy - returns the current horizontal velocity in cm/s
+     * get_speed_xy - returns the current horizontal speed in cm/s
      *
-     * @returns the current horizontal velocity in cm/s
+     * @returns the current horizontal speed in cm/s
      */
-    virtual float get_velocity_xy() const = 0;
+    virtual float get_speed_xy() const = 0;
 
     //
     // Z Axis methods

libraries/AP_InertialNav/AP_InertialNav_NavEKF.cpp

@@ -118,11 +118,11 @@ const Vector3f &AP_InertialNav_NavEKF::get_velocity() const
 }
 
 /**
- * get_velocity_xy - returns the current horizontal velocity in cm/s
+ * get_speed_xy - returns the current horizontal speed in cm/s
  *
- * @returns the current horizontal velocity in cm/s
+ * @returns the current horizontal speed in cm/s
  */
-float AP_InertialNav_NavEKF::get_velocity_xy() const
+float AP_InertialNav_NavEKF::get_speed_xy() const
 {
     return norm(_velocity_cm.x, _velocity_cm.y);
 }

libraries/AP_InertialNav/AP_InertialNav_NavEKF.h

@@ -76,11 +76,11 @@ public:
     float    get_pos_z_derivative() const;
 
     /**
-     * get_velocity_xy - returns the current horizontal velocity in cm/s
+     * get_speed_xy - returns the current horizontal speed in cm/s
      *
-     * @returns the current horizontal velocity in cm/s
+     * @returns the current horizontal speed in cm/s
      */
-    float        get_velocity_xy() const override;
+    float        get_speed_xy() const override;
 
     /**
      * get_altitude - get latest altitude estimate in cm