rename pg_print() as print_pg() and add it to to_str()

* NEWS: Mention those change.
* spot/twaalgos/game.hh (print_pg): New function.
(pg_print): Mark as deprecated.
* spot/twaalgos/game.cc (pg_print): Redirect to print_pg().
(print_pg): Update to output state names.
* python/spot/__init__.py: Teach to_str() about print_pg().
* bin/ltlsynt.cc: Adjust to call print_pg().
* tests/python/games.ipynb: Add an example.
* tests/core/ltlsynt.test: Adjust to remove the "INIT" note.

python/spot/__init__.py

@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-# Copyright (C) 2014-2021  Laboratoire de
+# Copyright (C) 2014-2022  Laboratoire de
 # Recherche et Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -261,6 +261,12 @@ class twa:
             ostr = ostringstream()
             print_lbtt(ostr, a, opt)
             return ostr.str()
+        if format == 'pg':
+            if opt is not None:
+                raise ValueError("print_pg() has no option")
+            ostr = ostringstream()
+            print_pg(ostr, a)
+            return ostr.str()
         raise ValueError("unknown string format: " + format)
 
     def save(a, filename, format='hoa', opt=None, append=False):