count() may be implemented using a loop, so using it touch
check count() == 1 or count() > 1 is not advisable.

* spot/twa/acc.hh (mark_t::is_singleton, mark_t::has_many): Introduce
these two methods to replace count()==1 and count()>1
* spot/twa/acc.cc, spot/twaalgos/cleanacc.cc,
spot/twaalgos/determinize.cc, spot/twaalgos/dtwasat.cc,
spot/twaalgos/iscolored.cc, spot/twaalgos/remfin.cc,
spot/twaalgos/toparity.cc: Adjust usage.