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.