fix ignored-qualifier warning

include/ALL_LB.hpp

@@ -160,7 +160,7 @@ public:
 
   /// method the get the number of vertices stored for the local domain
   /// @return number of local vertices
-  const int getNVertices() { return vertices.size(); }
+  int getNVertices() { return vertices.size(); }
 
   /// method to set undefined method specific data, which is not shared between
   /// different methods but needs to be set by a unified interface