This is not optimal yet because it still construct a minimal WDBA
internally, but it's better than the previous way to call
minimize_obligation() since it can avoid constructing the minimized
automaton in a few more cases.

* spot/tl/hierarchy.cc, spot/tl/hierarchy.hh: Introduce
is_obligation().
* bin/ltlfilt.cc: Wire it to --obligation.
* spot/twaalgos/minimize.cc: Implement is_wdba_realizable(),
needed by the above.
* tests/core/obligation.test: Test it.
* bin/man/spot-x.x, NEWS: Document it.