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.