introduce the original-classes named property

* doc/org/concepts.org, NEWS: Document it.
* spot/twaalgos/determinize.cc, spot/twaalgos/determinize.hh,
spot/twaalgos/sbacc.cc, spot/twaalgos/sbacc.hh: Use it.
* spot/twa/twagraph.cc: Update it on defrag.
* spot/twa/twa.cc (copy_named_properties_of): Copy it.
* tests/python/det.py: New file.
* tests/Makefile.am: Add it.
* python/spot/impl.i (get_original_states, get_original_classes): New
methods, to help with the tests.

tests/python/det.py

@@ -0,0 +1,35 @@
+#!/usr/bin/python3
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2021 Laboratoire de Recherche et Développement de
+# l'EPITA.
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import spot
+
+a = spot.translate('FGa | FGb')
+
+# The 8th argument requests the computation of original-classes.
+d = spot.tgba_determinize(a, False, True, True, True,
+                          None, -1, True)
+cld = list(d.get_original_classes())
+assert [0, 1, 2, 3, 3] == cld
+
+e = spot.sbacc(d)
+assert e.get_original_states() is None
+cle = list(e.get_original_classes())
+assert len(cle) == e.num_states()
+assert set(cle) == set(cld)