Algorithms that remove transitions can turn a non-deterministic
automaton into a deterministic one, so we need to be able to specify
that determinism can be improved (as opposed to preserved).

* spot/twa/twa.hh (twa::prop_set::improve_det): New attribute.
(twa::prop_keep, twa::prop_copy): Honor it.
* spot/tl/exclusive.cc, spot/twaalgos/alternation.cc,
spot/twaalgos/complete.cc, spot/twaalgos/degen.cc,
spot/twaalgos/determinize.cc, spot/twaalgos/mask.cc,
spot/twaalgos/minimize.cc, spot/twaalgos/remfin.cc,
spot/twaalgos/remprop.cc, spot/twaalgos/sbacc.cc,
spot/twaalgos/sccfilter.cc, spot/twaalgos/simulation.cc,
spot/twaalgos/strength.cc, spot/twaalgos/stutter.cc,
spot/twaalgos/totgba.cc: Adjust calls to prop_keep() and
prop_copy().