* src/tgbaalgos/stutter.cc: Add a new variant of Etessami's check,
closer to his original paper in IPL.
* src/ltltest/stutter.test: Add more tests.
* bench/stutter/user.sh: Include this new variant in the benchmark.
* src/ltltest/ltlfilt.test: Add more tests.
* src/ltltest/stutter.test: New test.
* src/ltltest/Makefile.am: Adjust.
* src/bin/ltlfilt.cc: Catch std::runtime_error.
* src/tgba/tgbasl.hh (make_tgbasl): New function.
* src/tgba/tgbagraph.hh (make_tgba_graph): Add another overload.
* src/tgbaalgos/stutter_invariance.cc,
src/tgbaalgos/stutter_invariance.hh: Take the algorithm version as an
optional integer, and call getenv() only once.
* bench/stutter/stutter_invariance_randomgraph.cc,
bench/stutter/stutter_invariance_formulas.cc: Simplify using the
above functions.
