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.

bin/randaut.cc

@@ -385,6 +385,8 @@ main(int argc, char** argv)
 
           if (opt_acceptance)
             aut->set_acceptance(accs, code);
+          if (aut->acc().is_t() || aut->acc().is_f())
+            aut->prop_weak(true);
 
           if (opt_uniq)
             {