specialized translation for GF(guarantee) and FG(safety)
This is adapted from a proposition in a paper by J. Esparza, J. Křentínský, and S. Sickert, submitted to LICS'18. We should add proper references to the code and documentation once that paper is accepted. * spot/twaalgos/gfguarantee.cc, spot/twaalgos/gfguarantee.hh: New files. * spot/twaalgos/Makefile.am, python/spot/impl.i: Add them. * spot/twa/fwd.hh: Add a forward declaration of bdd_dict_ptr. * spot/twaalgos/postproc.cc, spot/twaalgos/postproc.hh: Make it possible to call finalize() from the translator subclass. Constify all the do_* functions while we are there. * spot/twaalgos/translate.cc, spot/twaalgos/translate.hh: Add a "gf-guarantee" option to decide whether to use the new translation. * bin/spot-x.cc: Document it. * tests/core/dca2.test, tests/core/genltl.test, tests/core/ltl2tgba2.test, tests/core/parity2.test, tests/core/satmin.test, tests/python/automata.ipynb, tests/python/sbacc.py: Adjust test cases. * tests/python/except.py: Add a couple more tests.