This improves the determinism in a few cases.
* spot/twa/twagraph.cc (merge_edges): Encapsulate the two
passes into lambdas so that they are very easy to swap.
* spot/twa/twagraph.hh (merge_edges): Adjust documentation.
* tests/python/mergedge.py: Add test case.
* tests/core/alternating.test, tests/python/alternation.ipynb:
Determinism was improved.
* tests/core/parity2.test, tests/core/readsave.test,
tests/core/sbacc.test, tests/python/_product_susp.ipynb,
tests/python/atva16-fig2a.ipynb, tests/python/decompose.ipynb,
tests/python/highlighting.ipynb, tests/python/satmin.ipynb,
tests/python/simstate.py: Adjust expected order of edges.
* NEWS: Mention the change.
This fixes#441, reported by Jérôme Dubois.
* tests/python/mergedge.py: New file.
* tests/Makefile.am: Add it.
* spot/twa/twagraph.cc (merge_edges): Fix initialization of second
loop.
* NEWS: Mention the bug.
