zlktree: implement ACD and its transform

A quick and dirty implementation of the Alternating Cycle
Decomposition of the casares.21.icalp paper.

* spot/twaalgos/genem.cc, spot/twaalgos/genem.hh
(maximal_accepting_loops_for_scc): New function.
* spot/twaalgos/sccinfo.cc,
spot/twaalgos/sccinfo.hh (scc_and_mark_filter): Add a possibility to
specify a mask of transition to filter.
* spot/twaalgos/zlktree.hh, spot/twaalgos/zlktree.cc (acd): New class.
(acd_transform): New function.
* python/spot/__init__.py: Add SVG rendering for acd.
* tests/python/_zlktree.ipynb: Play with acd and acd_transform.
* tests/python/toparity.py: Add more tests to compare the
sizes of acd_transform and to_parity.
* NEWS: Mention this new feature.

python/spot/__init__.py

@@ -436,6 +436,14 @@ class zielonka_tree:
         self.dot(ostr)
         return _ostream_to_svg(ostr)
 
+@_extend(acd)
+class acd:
+    def _repr_svg_(self):
+        """Output the ACD as SVG"""
+        ostr = ostringstream()
+        self.dot(ostr)
+        return _ostream_to_svg(ostr)
+
 
 def automata(*sources, timeout=None, ignore_abort=True,
              trust_hoa=True, no_sid=False, debug=False,