improve fuse_marks_here by detecting more patterns
This remove some restrictions that prevented fuse_marks_here from simplifying certain patterns, as noted in the first comment of issue #405. * spot/twaalgos/cleanacc.cc (find_interm_rec, find_fusable): Remove some unnecessary restrictions to singleton marks, and replace the hack put one non-singleton mark at the beginning of the singleton list by a sort. * tests/python/simplacc.py: Add two test cases. * tests/python/automata.ipynb, tests/core/remfin.test: Improve expected results. * NEWS: Mention the bug.