spot::acc_cond::mark_t is implemented as a bit vector using a single
unsigned, and implicit conversions between mark_t and unsigned may be
confusing. We try to use the proper interface.

* bin/autfilt.cc, bin/ltlsynt.cc, spot/kripke/kripke.cc,
  spot/misc/game.hh, spot/parseaut/parseaut.yy, spot/priv/accmap.hh,
  spot/ta/ta.cc, spot/ta/taexplicit.cc, spot/ta/taproduct.cc,
  spot/taalgos/emptinessta.cc, spot/taalgos/tgba2ta.cc, spot/twa/acc.cc,
  spot/twa/acc.hh, spot/twa/taatgba.cc, spot/twa/taatgba.hh,
  spot/twa/twagraph.hh, spot/twaalgos/alternation.cc,
  spot/twaalgos/cleanacc.cc, spot/twaalgos/cobuchi.cc,
  spot/twaalgos/complete.cc, spot/twaalgos/couvreurnew.cc,
  spot/twaalgos/degen.cc, spot/twaalgos/dot.cc,
  spot/twaalgos/dtwasat.cc, spot/twaalgos/dualize.cc,
  spot/twaalgos/emptiness.cc, spot/twaalgos/gtec/ce.cc,
  spot/twaalgos/gtec/gtec.cc, spot/twaalgos/gtec/sccstack.cc,
  spot/twaalgos/gv04.cc, spot/twaalgos/hoa.cc, spot/twaalgos/lbtt.cc,
  spot/twaalgos/ltl2tgba_fm.cc, spot/twaalgos/magic.cc,
  spot/twaalgos/ndfs_result.hxx, spot/twaalgos/rabin2parity.cc,
  spot/twaalgos/randomgraph.cc, spot/twaalgos/remfin.cc,
  spot/twaalgos/sbacc.cc, spot/twaalgos/sccfilter.cc,
  spot/twaalgos/sccinfo.cc, spot/twaalgos/sccinfo.hh,
  spot/twaalgos/se05.cc, spot/twaalgos/sepsets.cc,
  spot/twaalgos/simulation.cc, spot/twaalgos/strength.cc,
  spot/twaalgos/stripacc.cc, spot/twaalgos/stutter.cc,
  spot/twaalgos/sum.cc, spot/twaalgos/tau03.cc,
  spot/twaalgos/tau03opt.cc, spot/twaalgos/totgba.cc,
  spot/twaalgos/toweak.cc, python/spot/impl.i, tests/core/acc.cc,
  tests/core/twagraph.cc: do not confuse mark_t and unsigned
* tests/python/acc_cond.ipynb: warn about possible change of the API