Fix danging reference bug caused by halfedge().from_vertex().

We should not return references to members of possible xvalues.
Handle is lightweight enough that copying is not a problem.

src/OpenVolumeMesh/Core/BaseEntities.hh

@@ -61,10 +61,10 @@ public:
     virtual ~OpenVolumeMeshEdge() {
     }
 
-    const VertexHandle& from_vertex() const {
+    const VertexHandle from_vertex() const {
         return fromVertex_;
     }
-    const VertexHandle& to_vertex() const {
+    const VertexHandle to_vertex() const {
         return toVertex_;
     }