This is a huge patch.  tgba_digraph are equiped with some boolean
properties that can be used to indicate whether they represent SBA
(and will carry more informations later).  All algorithms that produce
or use sba_explicit_* automata are changed to use tgba_digraph.
postproc has been rewritten using only tgba_digraph, and this required
changing the return types of many algorithms from tgba* to
tgba_digraph*.

* src/bin/dstar2tgba.cc, src/bin/ltlfilt.cc, src/dstarparse/dra2ba.cc,
src/dstarparse/dstar2tgba.cc, src/dstarparse/nra2nba.cc,
src/dstarparse/nsa2tgba.cc, src/dstarparse/public.hh,
src/tgba/tgbagraph.hh, src/tgba/tgbasafracomplement.cc,
src/tgbaalgos/compsusp.cc, src/tgbaalgos/compsusp.hh,
src/tgbaalgos/degen.cc, src/tgbaalgos/degen.hh,
src/tgbaalgos/dotty.cc, src/tgbaalgos/minimize.cc,
src/tgbaalgos/minimize.hh, src/tgbaalgos/postproc.cc,
src/tgbaalgos/postproc.hh, src/tgbaalgos/sccfilter.cc,
src/tgbaalgos/sccinfo.cc, src/tgbaalgos/stripacc.cc,
src/tgbaalgos/stripacc.hh, src/tgbaalgos/translate.cc,
src/tgbaalgos/translate.hh, src/tgbatest/ltl2tgba.cc,
wrap/python/spot.i: Update.