specialized translation for GF(guarantee) and FG(safety)

This is adapted from a proposition in a paper by J. Esparza,
J. Křentínský, and S. Sickert, submitted to LICS'18.  We should add
proper references to the code and documentation once that paper is
accepted.

* spot/twaalgos/gfguarantee.cc, spot/twaalgos/gfguarantee.hh:
New files.
* spot/twaalgos/Makefile.am, python/spot/impl.i: Add them.
* spot/twa/fwd.hh: Add a forward declaration of bdd_dict_ptr.
* spot/twaalgos/postproc.cc, spot/twaalgos/postproc.hh: Make it
possible to call finalize() from the translator subclass.  Constify
all the do_* functions while we are there.
* spot/twaalgos/translate.cc, spot/twaalgos/translate.hh: Add
a "gf-guarantee" option to decide whether to use the new translation.
* bin/spot-x.cc: Document it.
* tests/core/dca2.test, tests/core/genltl.test,
tests/core/ltl2tgba2.test, tests/core/parity2.test,
tests/core/satmin.test, tests/python/automata.ipynb,
tests/python/sbacc.py: Adjust test cases.
* tests/python/except.py: Add a couple more tests.

tests/python/except.py

@@ -75,3 +75,15 @@ try:
     a = r.reduce()
 except RuntimeError as e:
     assert "empty cycle" in str(e)
+
+f = spot.formula('GF(a | Gb)')
+try:
+    spot.gf_guarantee_to_ba(f, spot._bdd_dict)
+except RuntimeError as e:
+    assert "guarantee" in str(e)
+
+f = spot.formula('FG(a | Fb)')
+try:
+    spot.fg_safety_to_dca(f, spot._bdd_dict)
+except RuntimeError as e:
+    assert "safety" in str(e)

tests/python/sbacc.py

@@ -1,5 +1,5 @@
 # -*- mode: python; coding: utf-8 -*-
-# Copyright (C) 2017 Laboratoire de Recherche et Développement de l'Epita
+# Copyright (C) 2017, 2018 Laboratoire de Recherche et Développement de l'Epita
 # (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -50,12 +50,12 @@ h = s.to_str('hoa')
 assert h == """HOA: v1
 States: 2
 Start: 0
-AP: 2 "b" "a"
+AP: 2 "a" "b"
 Acceptance: 2 Inf(0) | Inf(1)
 properties: trans-labels explicit-labels state-acc deterministic
 --BODY--
 State: 0
-[1] 1
+[0] 1
 State: 1 {1}
 [t] 1
 --END--"""
@@ -84,13 +84,13 @@ h = d.to_str('hoa')
 assert h == """HOA: v1
 States: 2
 Start: 0
-AP: 2 "b" "a"
+AP: 2 "a" "b"
 acc-name: Buchi
 Acceptance: 1 Inf(0)
 properties: trans-labels explicit-labels state-acc deterministic
 --BODY--
 State: 0
-[1] 1
+[0] 1
 State: 1 {0}
 [t] 1
 --END--"""