Convert a formula into a string parsable by Spin.
* src/tgbaalgos/neverclaim.hh, src/tgbaalgos/neverclaim.cc: New files.
Print the never claim in Spin format of a degeneralized TGBA.
* src/tgbaalgos/Makefile.am: Add them.
* src/tgbatest/ltl2tgba.cc: Add the option -N which outputs the
never claim in Spin format of a degeneralized TGBA.
* src/tgbatest/ltl2neverclaim.test: New file.
* src/tgbatest/Makefile.am: Add it.
