improve acceptance simplifications using useless colors

This fixes issue #418.

* spot/twa/acc.cc,
spot/twa/acc.hh (acc_cond::acc_code::useless_colors_patterns): New
method to detect patterns of colors allowing other colors to be added
or removed at will.
* spot/twaalgos/cleanacc.cc (simplify_acceptance_here): Use the above
patterns to remove some useless colors from transitions and hope that
this can help simplify the acceptance condition.
* spot/twaalgos/degen.cc (propagate_marks_vector): Use the pattern to
add more colors.
* tests/core/ltl2tgba2.test: Add the test case from issue #418.
* tests/core/ltl2dstar4.test, tests/core/satmin3.test,
tests/core/sccdot.test, tests/core/sim3.test,
tests/python/automata.ipynb, tests/python/decompose.ipynb,
tests/python/merge.py, tests/python/pdegen.py, tests/python/remfin.py,
tests/python/toparity.py, tests/python/tra2tba.py: Adjust all test
cases.
* NEWS: Mention this new feature.

NEWS

@@ -56,6 +56,13 @@ New in spot 2.9.3.dev  (not yet released)
     the form Inf(x)&Inf(y) or Fin(x)|Fin(y) when the presence of x
     always implies that of y.  (Issue #406.)
 
+  - simplifiy_acceptance_here() and propagate_marks_here() learned to
+    use some patterns of the acceptance condition to remove or add
+    (depending on the function) colors on edges.  As an example, with
+    a condition such as Inf(0) | Fin(1), an edge labeled by {0,1} can
+    be simplied to {0}, but the oppose rewriting can be useful as
+    well.  (Issue #418.)
+
   Bugs fixed:
 
   - Handle xor and <-> in a more natural way when translating