Math: add Vector2 is_zero method

libraries/AP_Math/vector2.h

@@ -105,6 +105,9 @@ struct Vector2
     // check if any elements are infinity
     bool is_inf(void) const;
 
+    // check if all elements are zero
+    bool is_zero(void) const { return (fabsf(x) < FLT_EPSILON) && (fabsf(y) < FLT_EPSILON); }
+
     // zero the vector
     void zero()
     {