Add a decompose_scc() function

See #172.
While at it, fix typo in doxygen comment.

* spot/twaalgos/strength.cc, spot/twaalgos/strength.hh: New function.
* tests/python/decompose_scc.py, tests/Makefile.am: Test python
binding.

* spot/twaalgos/mask.hh: Fix typo.

tests/python/decompose_scc.py

@@ -0,0 +1,51 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2017 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
+
+aut = spot.translate('(Ga -> Gb) W c')
+
+si = spot.scc_info(aut)
+
+assert (spot.decompose_scc(si, 2).to_str('hoa', '1.1') == """HOA: v1.1
+States: 3
+Start: 0
+AP: 3 "b" "a" "c"
+acc-name: Buchi
+Acceptance: 1 Inf(0)
+properties: trans-labels explicit-labels state-acc !complete
+properties: deterministic
+--BODY--
+State: 0
+[!1&!2] 0
+[1&!2] 1
+State: 1
+[!1&!2] 0
+[1&!2] 1
+[1&2] 2
+State: 2
+[1] 2
+--END--""")
+
+try:
+    spot.decompose_scc(si, 4)
+except ValueError:
+    pass
+else:
+    raise AssertionError