not create Now/Next variable when G is the root of the formula.
(ltl_trad_visitor::ltl_trad_visitor): Take a root argument.
(ltl_trad_visitor::recurse): Create a new visitor, do not copy
the current one.
(ltl_to_tgba): Build ltl_trad_visitor with root = true.

* src/tgbaalgos/ltl2tgba.cc (ltl_trad_visitor::visit(unop::X)):
Remove FIXME about handling X(a U b) and X(a R b) better, it's
done naturally.