AP_Math: make is_nan const for quaternion and add .zero() for vector2

libraries/AP_Math/quaternion.h

@@ -47,7 +47,7 @@ public:
     }
 
     // check if any elements are NAN
-    bool        is_nan(void)
+    bool        is_nan(void) const
     {
         return isnan(q1) || isnan(q2) || isnan(q3) || isnan(q4);
     }

libraries/AP_Math/vector2.h

@@ -105,6 +105,12 @@ struct Vector2
     // check if any elements are infinity
     bool is_inf(void) const;
 
+    // zero the vector
+    void zero()
+    {
+        x = y = 0;
+    }
+
     // gets the length of this vector squared
     T   length_squared() const
     {