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().

python/spot/impl.i

@@ -389,6 +389,7 @@ namespace std {
 %include <spot/graph/graph.hh>
 %nodefaultctor spot::digraph;
 %nodefaultctor spot::internal::state_out;
+%nodefaultctor spot::internal::all_trans;
 %traits_swigtype(spot::internal::edge_storage<unsigned int, unsigned int, unsigned int, spot::internal::boxed_label<spot::twa_graph_edge_data, false> >);
 %fragment(SWIG_Traits_frag(spot::internal::edge_storage<unsigned int, unsigned int, unsigned int, spot::internal::boxed_label<spot::twa_graph_edge_data, false> >));
 
@@ -397,10 +398,17 @@ namespace std {
 			       SWIG_POINTER_OWN);
 }
 
+%typemap(edges, optimal="1") spot::internal::all_trans<spot::digraph<spot::twa_graph_state, spot::twa_graph_edge_data>> {
+  $result = SWIG_NewPointerObj(new $1_ltype($1), $&1_descriptor,
+			       SWIG_POINTER_OWN);
+}
+
 %noexception spot::twa_graph::out;
+%noexception spot::twa_graph::edges;
 %include <spot/twa/twagraph.hh>
 
 %template(twa_graph_state_out) spot::internal::state_out<spot::digraph<spot::twa_graph_state, spot::twa_graph_edge_data>>;
+%template(twa_graph_all_trans) spot::internal::all_trans<spot::digraph<spot::twa_graph_state, spot::twa_graph_edge_data>>;
 %template(twa_graph_edge_boxed_data) spot::internal::boxed_label<spot::twa_graph_edge_data, false>;
 %template(twa_graph_edge_storage) spot::internal::edge_storage<unsigned int, unsigned int, unsigned int, spot::internal::boxed_label<spot::twa_graph_edge_data, false> >;
 
@@ -582,6 +590,14 @@ namespace std {
    }
 }
 
+%extend spot::internal::all_trans<spot::digraph<spot::twa_graph_state, spot::twa_graph_edge_data>> {
+  swig::SwigPyIterator* __iter__(PyObject **PYTHON_SELF)
+   {
+      return swig::make_forward_iterator(self->begin(), self->begin(),
+				         self->end(), *PYTHON_SELF);
+   }
+}
+
 %extend spot::acc_cond::acc_code {
   std::string __repr__()
   {

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
+    internal::all_trans<const graph_t>
-      SPOT_RETURN(g_.edges());
+    edges() const
-    auto edges()
+    {
-      SPOT_RETURN(g_.edges());
+      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()