Return a state*, not a state_bdd.
* src/tgba/succiterconcrete.hh
(tgba_succ_iterator_concrete::current_state): Return a state_bdd*,
not a state_bdd.
* src/tgba/state.hh (state::as_bdd): New abstract method.
* src/tgba/statebdd.hh (state_bdd::as_bdd): Move definitions ...
* src/tgba/statebdd.cc (state_bdd::as_bdd): ... here.
* src/tgba/tgba.hh: Add Doxygen comments.
(tgba::succ_iter, tgba::get_init_state): Use state*, not state_bdd.
* src/tgba/tgbabddconcrete.hh (tgba_bdd_concrete::get_init_state):
Return a state_bdd*, not a state_bdd.
(tgba_bdd_concrete::get_init_bdd): New method.
(tgba_bdd_concrete::succ_uter): Take a state* as argument.
* src/tgba/tgbabddconcrete.cc: Likewise.
* src/tgba/tgbabddtranslatefactory.cc
(tgba_bdd_translate_factory::tgba_bdd_translate_factory): Use
tgba_bdd_concrete::get_init_bdd.
* src/tgbaalgos/dotty.cc (dotty_state, dotty_rec, dotty): Adjust
to use state* instead of state_bdd.
* src/tgba/succlist.hh: Delete.  (Leftover from a previous
draft.)