This fixes #6.

* src/graph/graph.hh: Rename the old transitions() as
transition_vector(), and implement a new transitions() that iterates
only on non-dead transitions.
* src/tgba/tgbagraph.hh, src/tgba/tgbagraph.cc, src/graph/ngraph.hh:
Adjust wrappers.
* src/hoaparse/hoaparse.yy, src/tgbaalgos/complete.cc,
src/tgbaalgos/dtbasat.cc, src/tgbaalgos/dtgbasat.cc,
src/tgbaalgos/randomize.cc, src/tgbaalgos/safety.cc: Adjust calls.