* spot/tl/hierarchy.hh, spot/tl/hierarchy.cc (nesting_depth): New
function.
* python/spot/__init__.py: Also make it a method of formula in Python
* bin/common_output.cc, bin/common_output.hh: Implement
--stats=%[OP]n.
* NEWS: Mention it.
* tests/core/format.test, tests/python/formulas.ipynb: Test it.
* doc/footer.html: Make the footer XML compatible.
* doc/mainpage.dox: Fix references to modules.
* spot/tl/formula.hh: Introduce a hierarchy module.
* spot/tl/hierarchy.hh: Use it.
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.
* bin/common_output.cc: Move some of the printing code...
* spot/tl/hierarchy.cc, spot/tl/hierarchy.hh: ... here, as new
variants of mp_class...
* python/spot/impl.i: ... that we can now call from Python.
* python/ajax/spotcgi.in: Use those to simplify and extend
the code printing class membership.
Tools for deciding the class of a formula.
* spot/tl/hierarchy.cc, spot/tl/hierarchy.hh: New files.
* spot/tl/Makefile.am: Add them.
* bin/common_output.cc, bin/common_output.hh: Implement --format=%h.
* tests/core/hierarchy.test: More tests.
* NEWS: Update.
