Combined with 87c2b29, this fixes#7.
* src/tgbaalgos/stutterize.cc: Call keep_props().
* src/tgbaalgos/closure.cc: Just specify the encoding.
* src/bin/autfilt.cc: Add a --instut=2 option.
* src/tgbatest/stutter.test: More test.
* src/bin/autfilt.cc: Add these new options.
* src/tgbaalgos/stutterize.cc, src/tgbaalgos/stutterize.hh: Make it
possible to call sl() and sl2() without passing the set of atomic
propositions.
* src/tgbatest/stutter.test: New file.
* src/tgbatest/Makefile.am: Add it.