new keyword in the ELTL format: finish, which applies to an
automaton operator and tells whether it just completed.
* src/eltlparse/eltlparse.yy: Clean it. Add finish.
* src/eltlparse/eltlscan.ll: Add finish.
* src/formula_tree.cc, src/formula_tree.hh: New files. Define a
small AST representing formulae where atomic props are unknown
which is used in the ELTL parser.
* src/ltlast/automatop.cc, ltlast/automatop.hh, ltlast/nfa.cc,
ltlast/nfa.hh: Adjust.
* src/ltlast/unop.cc, src/ltlast/unop.hh: Finish is an unop.
* src/ltlvisit/basicreduce.cc, src/ltlvisit/nenoform.cc,
src/ltlvisit/reduce.cc, src/ltlvisit/syntimpl.cc,
src/ltlvisit/tostring.cc, src/ltlvisit/tunabbrev.cc,
src/tgba/formula2bdd.cc, src/tgbaalgos/ltl2tgba_fm.cc,
src/tgbaalgos/ltl2tgba_lacim.cc: Handle finish in switches.
* src/tgbaalgos/eltl2tgba_lacim.cc: Translate finish.
* src/tgbatest/eltl2tgba.test: More tests.
