* NEWS, spot/twa/twa.hh: Document the change.
* spot/twa/twagraph.hh, spot/kripke/kripkegraph.hh:
Add an exception in get_init_state_number().
get_init_state() now calls get_init_state_number().
* spot/twa/twagraph.cc, spot/twaalgos/simulation.cc,
spot/twaalgos/powerset.cc, spot/twaalgos/complete.cc,
spot/twaalgos/sccfilter.cc: Remove now useless tests.
* spot/twaalgos/hoa.cc: Remove now useless comment.
* spot/twaalgos/minimize.cc: Never return an automaton with no state.