ltl_to_tgba_fm: Build a tgba_digraph instead of a tgba_explicit_formula
The conversion is not complete, because the conversion from SERE to DRA used for the closure operator is still building a tgba_explicit_formula. * src/tgbaalgos/ltl2tgba_fm.cc, src/tgbaalgos/ltl2tgba_fm.hh: Return a tgba_digraph. * src/priv/acccompl.cc: Simplify. * src/graph/ngraph.hh: Add a way to iterate over all names. * src/tgba/tgbagraph.hh (compute_support_conditions): Return something useful. It's actually used by the constructor of testing automata. * src/tgbatest/wdba.test: Adjust to the fact that state are not labeled by formulas anymore. * src/bin/ltl2tgba.cc, src/bin/ltl2tgta.cc: Do not try to enable UTF8 on automata anymore.