python: add wrapper on twa_graph::edges()

* spot/twa/twagraph.hh (edges): Do not hide from SWIG.
* spot/graph/graph.hh: Hide stuff that SWIG do not understand.
* python/spot/impl.i: Add some typemaps and fragment to
iterate over the result of twa_graph::edges().

spot/graph/graph.hh

@@ -527,11 +527,13 @@ namespace spot
     template <typename Graph>
     class SPOT_API all_trans
     {
+    public:
       typedef typename std::conditional<std::is_const<Graph>::value,
 					const typename Graph::edge_vector_t,
 					typename Graph::edge_vector_t>::type
 	tv_t;
       typedef all_edge_iterator<Graph> iter_t;
+    private:
       tv_t& tv_;
     public:
 

spot/twa/twagraph.hh

@@ -408,11 +408,20 @@ namespace spot
     auto states()
       SPOT_RETURN(g_.states());
 
-    auto edges() const
-      SPOT_RETURN(g_.edges());
-    auto edges()
-      SPOT_RETURN(g_.edges());
+    internal::all_trans<const graph_t>
+    edges() const
+    {
+      return g_.edges();
+    }
+#endif
 
+    internal::all_trans<graph_t>
+    edges()
+    {
+      return g_.edges();
+    }
+
+#ifndef SWIG
     auto edge_vector() const
       SPOT_RETURN(g_.edge_vector());
     auto edge_vector()