and make sure are_isomorphic does not look only at the number of
acceptance sets

* src/tgba/acc.hh: Here.
* src/tgbaalgos/are_isomorphic.cc: Use it to ensure two automata
have the same acceptance condition.
* src/tgbatest/explpro4.test: Test product between Büchi and co-Büchi,
and make sure the isomorphic check look at the acceptance condition.