spot.complete() could complete an empty co-Büchi automaton into an
automaton accepting everything.
* NEWS: Document it
* spot/twaalgos/complete.cc: Fix it
* tests/core/complete.test, tests/core/prodor.test: Test it
and call it after parsing
* spot/twa/twagraph.cc, spot/twa/twagraph.hh
(twa_graph::merge_univ_dests): New method.
* spot/parseaut/parseaut.yy: Call it.
* spot/twaalgos/dot.cc: Improve output, now that
several edges can use the same universal destination.
* tests/core/alternating.test, tests/core/complete.test,
tests/core/parseaut.test, tests/python/_altscc.ipynb,
tests/python/alternating.py, tests/python/alternation.ipynb: Adjust
test case.
* doc/org/tut24.org: Adjust example.
Alexandre Lewkowicz reported a case where complete() would peek an
existing state that is accepting, and wrongly use it as a sink.
* spot/twaalgos/complete.cc: Fix the function.
* tests/core/complete.test: Add two more tests.
* NEWS: Mention the bug.
