twa_graph: fix purge_unreachable_states on alternating automata
The algorithm had two problems: it was removing only useless destination from universal destination (instead of removing the entire edge), and it was not properly iterating over the entire reachable automaton. * spot/twa/twagraph.cc: Fix it. * spot/twa/twagraph.hh: Adjust documentation. * tests/core/alternating.test: Add more tests. * tests/python/twagraph.py: Adjust. * NEWS: Mention the bug.