Fix label-parallel-edges property type

It should be 'size_t' instead of 'long'.

src/graph/graph.cc

@@ -584,7 +584,7 @@ void GraphInterface::LabelParallelEdges(string property)
     }
     catch (property_not_found)
     {
-        typedef vector_property_map<long,edge_index_map_t> parallel_map_t;
+        typedef vector_property_map<size_t,edge_index_map_t> parallel_map_t;
         parallel_map_t parallel_map(_edge_index);
         check_filter(*this, bind<void>(label_parallel_edges(), _1, _edge_index, parallel_map), reverse_check(), directed_check());
         _properties.property(property, parallel_map);