If the input is a tgba_explicit_formula we can output a
tgba_explicit_formula too, and we want to do that because it is
more space efficient.

* src/tgba/tgbaexplicit.hh (get_label): New method.
* src/tgbaalgos/sccfilter.cc (create_transition): New function,
to handle tgba_explicit_formula and tgba_explicit_string output
differently.
(filter_iter): Template it on the output tgba type, and adjust
to call create_transition.
(scc_filter): Use filter_iter<tgba_explicit_formula> or
filter_iter<tgba_explicit_string> depending on the input tgba
type.