python: give access to the "product-states" property

* python/spot/impl.i (get_product_states, set_product_states): New.
* tests/python/product.ipynb: Use it.
* NEWS: Mention it.

NEWS

@@ -212,6 +212,11 @@ New in spot 2.4.3.dev (not yet released)
   - The new function spot::check_determinism() sets both
     prop_semi_deterministic() and prop_universal() appropriately.
 
+  Python:
+
+  - The "product-states" property of automata is now accessible via
+    spot.twa.get_product_states() and spot.set.get_product_states().
+
   Deprecation notices:
 
   (These functions still work but compilers emit warnings.)

python/spot/impl.i

@@ -428,8 +428,10 @@ namespace swig
 
 namespace std {
   %template(liststr) list<std::string>;
+  %template(pairunsigned) pair<unsigned, unsigned>;
   %template(vectorformula) vector<spot::formula>;
   %template(vectorunsigned) vector<unsigned>;
+  %template(vectorpairunsigned) vector<pair<unsigned, unsigned>>;
   %template(vectorbool) vector<bool>;
   %template(vectorbdd) vector<bdd>;
   %template(vectorstring) vector<string>;
@@ -465,6 +467,19 @@ namespace std {
        DeprecationWarning)
 %}
 
+// Must occur before the twa declaration
+%typemap(out) SWIGTYPE* spot::twa::get_product_states %{
+  if (!$1)
+    $result = SWIG_Py_Void();
+  else
+    {
+      unsigned sz = $1->size();
+      $result = PyList_New(sz);
+      for (unsigned i = 0; i < sz; ++i)
+        PyList_SetItem($result, i, swig::from((*$1)[i]));
+    }
+%}
+
 %include <spot/twa/twa.hh>
 
 %include <spot/tl/apcollect.hh>
@@ -769,6 +784,21 @@ def state_is_accepting(self, src) -> "bool":
     return self->get_named_prop<std::vector<std::string>>("state-names");
   }
 
+
+  void set_product_states(std::vector<std::pair<unsigned, unsigned>> pairs)
+  {
+    self->set_named_prop("product-states", new
+                         std::vector<std::pair<unsigned,
+                                               unsigned>>(std::move(pairs)));
+  }
+
+  std::vector<std::pair<unsigned, unsigned>>* get_product_states()
+  {
+    return self->get_named_prop
+      <std::vector<std::pair<unsigned, unsigned>>>("product-states");
+  }
+
+
   twa* highlight_state(unsigned state, unsigned color)
   {
     auto hs =