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.

NEWS

@@ -117,6 +117,17 @@ New in spot 2.10.6.dev  (not yet released)
     to obtain a simple model checker (that returns true or false,
     without counterexample).
 
+ Python bindings:
+
+  - The to_str() method of automata can now export a parity game into
+    the PG-Solver format by passing option 'pg'.  See
+    https://spot.lrde.epita.fr/ipynb/games.html for an example.
+
+ Deprectation notice:
+
+  - spot::pg_print() has been deprecated in favor of spot::print_pg()
+    for consistency with the rest of the API.
+
   Bugs fixed:
 
   - calling twa_graph::new_univ_edge(src, begin, end, cond, acc) could