* src/tgbaalgos/dupexp.cc, src/tgbaalgos/dupexp.hh: Produce
a tgba_digraph instead of a tgba_explicit_number.
* src/tgbaalgos/simulation.cc: First pass to adjust to the use of
tgba_digraph as a return of tgba_dupexp_dfs() and tgba_dupexp_bfs().
Some maps have been replaced by vectors because states are indexed,
but more simplifications could be done.