fix several algorithms that incorrectly preserved !weak

This massive set of changes was triggered by issue #546.
In addition to the better handling of !weak, this also adds some
weak properties in a few places.

* spot/twaalgos/product.cc (product_aux): Throw some exception
if an automaton with t or f acceptance has the !weak property.  This
is a cheap sanity check to help detect algorithms that incorrectly
assumed !weak input would necessarily become !weak output.
* spot/twaalgos/hoa.cc (print_hoa): Likewise, also do not assume
that terminal implies very-weak.
* spot/parseaut/parseaut.yy: Add several diagnostics for similar
cases.  E.g., a one-state automaton cannot be declared as !very-weak.
* tests/core/parseaut.test: Check those new diagnostics.
* spot/twa/twa.cc (twa::intersecting_run): Temporary remove the weak
property by setting it to maybe, not to false.
* spot/twaalgos/minimize.cc, spot/twaalgos/parity.cc,
spot/twaalgos/sccfilter.cc, spot/twaalgos/simulation.cc: Account for
the fact that these algorithm may in fact improve the weakness.
* spot/twaalgos/strength.cc: Only look at colors used by the
acceptance condition when deciding weakness.
* spot/twaalgos/synthesis.cc: Declare the strategy as weak.
* bin/randaut.cc: Add weak to automata with t/f acceptance.
* spot/kripke/kripke.hh: Make kripke structures as weak.
* tests/core/acc_word.test, tests/core/alternating.test,
tests/core/complement.test, tests/core/complete.test,
tests/core/ltlsynt.test, tests/core/randomize.test,
tests/core/readsave.test, tests/core/remfin.test,
tests/core/sccsimpl.test, tests/core/strength.test,
tests/core/wdba2.test, tests/ltsmin/kripke.test,
tests/python/automata-io.ipynb, tests/python/automata.ipynb,
tests/python/dbranch.py, tests/python/highlighting.ipynb,
tests/python/kripke.py, tests/python/ltsmin-dve.ipynb,
tests/python/mealy.py, tests/python/simstate.py: Adjust all these test
cases.
* NEWS: Mention the fixes.

tests/core/parseaut.test

@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2014-2018, 2020-2022 Laboratoire de Recherche et
+# Copyright (C) 2014-2018, 2020-2023 Laboratoire de Recherche et
 # Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -664,7 +664,7 @@ Start: 0
 AP: 1 "a"
 acc-name: all
 Acceptance: 0 t
-properties: trans-labels explicit-labels state-acc deterministic
+properties: trans-labels explicit-labels state-acc deterministic weak
 --BODY--
 State: 0
 [0] 1
@@ -1322,7 +1322,7 @@ AP: 2 "a" "b"
 acc-name: all
 Acceptance: 0 t
 properties: trans-labels explicit-labels state-acc complete
-properties: deterministic
+properties: deterministic weak
 --BODY--
 State: 0
 [0&!1] 0
@@ -1778,7 +1778,7 @@ Start: 0
 AP: 1 "a"
 Acceptance: 1 t
 properties: trans-labels explicit-labels trans-acc complete
-properties: deterministic
+properties: deterministic weak
 --BODY--
 State: 0 "F(a)"
 [0] 1 {0}
@@ -1844,7 +1844,7 @@ AP: 1 "a"
 acc-name: all
 Acceptance: 0 t
 properties: trans-labels explicit-labels state-acc complete
-properties: deterministic
+properties: deterministic weak
 --BODY--
 State: 0 "F(a)"
 [0] 1
@@ -2079,6 +2079,15 @@ properties: complete !weak very-weak
 --BODY--
 State: 0 0
 --END--
+HOA: v1
+States: 1
+Start: 0
+AP: 0
+Acceptance: 0 t
+properties: complete !very-weak
+--BODY--
+State: 0 0
+--END--
 EOF
 
 expecterr input <<EOF
@@ -2091,14 +2100,18 @@ input:36.36-43: 'properties: terminal' contradicts...
 input:36.13-28: ... 'properties: !inherently-weak' given here
 input:36.36-43: 'properties: terminal' contradicts...
 input:36.30-34: ... 'properties: !weak' given here
+input:36.30-34: an automaton with this condition is necessarily weak
+input:36.13-28: an automaton with this condition is necessarily inherently-weak
 input:46.13-28: 'properties: !inherently-weak' contradicts...
 input:46.35-43: ... 'properties: very-weak' given here
 input:46.30-33: 'properties: weak' contradicts...
 input:46.13-28: ... 'properties: !inherently-weak' given here
 input:55.22-26: 'properties: !weak' contradicts...
 input:55.28-36: ... 'properties: very-weak' given here
+input:55.22-26: an automaton with this condition is necessarily weak
 input:51.1-9: state 1 is unused and undefined
 input:55.13-20: automaton is incomplete because it has undefined states
+input:64.22-31: an automaton with 0 or 1 state is necessarily very-weak
 EOF