and adjust it to detect siPSL formulas, as in the paper of Dax et
al. (ATVA'09).  For issue #51.

* src/ltlast/atomic_prop.cc, src/ltlast/binop.cc,
src/ltlast/bunop.cc, src/ltlast/constant.cc, src/ltlast/formula.cc,
src/ltlast/formula.hh, src/ltlast/multop.cc, src/ltlast/unop.cc: Rename
the property, and adjust its computation on siSERE.
* src/ltlvisit/remove_x.cc, src/ltlvisit/simplify.cc,
src/tgbaalgos/stutter.cc: Adjust to new names.
* src/bin/ltlfilt.cc: Add option --syntactic-sutter-invariant.
* src/ltltest/kind.test: Update tests and add some new.