introduce a zielonka_tree class

* spot/twaalgos/zlktree.cc, spot/twaalgos/zlktree.hh: New files.
* spot/twaalgos/Makefile.am: Add them.
* tests/python/_zlktree.ipynb: New file.
* tests/Makefile.am: Add it.
* python/spot/__init__.py, python/spot/impl.i: Add bindings for it.
* doc/spot.bib (casares.21.icalp): New entry.
* NEWS: Mention this.

doc/spot.bib

@@ -1,4 +1,3 @@
-
 @InProceedings{	  babiak.12.tacas,
   author	= {Tom{\'a}{\v{s}} Babiak and Mojm{\'i}r
 		  K{\v{r}}et{\'i}nsk{\'y} and Vojt{\v{e}}ch {\v{R}}eh{\'a}k
@@ -114,6 +113,24 @@
   url		= {http://www.numdam.org/item/ITA_1999__33_6_495_0/}
 }
 
+@InProceedings{	  casares.21.icalp,
+  author	= {Antonio Casares and Thomas Colcombet and Nathana\"{e}l
+		  Fijalkow},
+  title		= {Optimal Transformations of Games and Automata Using
+		  {M}uller Conditions},
+  booktitle	= {Proceedings of the 48th International Colloquium on
+		  Automata, Languages, and Programming (ICALP'21)},
+  pages		= {123:1--123:14},
+  series	= {Leibniz International Proceedings in Informatics
+		  (LIPIcs)},
+  year		= {2021},
+  volume	= {198},
+  editor	= {Bansal, Nikhil and Merelli, Emanuela and Worrell, James},
+  publisher	= {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
+  address	= {Dagstuhl, Germany},
+  doi		= {10.4230/LIPIcs.ICALP.2021.123}
+}
+
 @InProceedings{	  cerna.03.mfcs,
   author	= {Ivana {\v{C}}ern{\'a} and Radek Pel{\'a}nek},
   title		= {Relating Hierarchy of Temporal Properties to Model
@@ -169,13 +186,13 @@
   doi		= {10.1109/TCAD.2008.2003303}
 }
 
-@article{   clemente.2.17.corr,
-  author    = {Lorenzo Clemente and Richard Mayr},
-  title     = {Efficient reduction of nondeterministic automata with application
-               to language inclusion testing},
-  journal   = {CoRR},
-  volume    = {abs/1711.09946},
-  year      = {2017},
+@Article{	  clemente.2.17.corr,
+  author	= {Lorenzo Clemente and Richard Mayr},
+  title		= {Efficient reduction of nondeterministic automata with
+		  application to language inclusion testing},
+  journal	= {CoRR},
+  volume	= {abs/1711.09946},
+  year		= {2017}
 }
 
 @Article{	  courcoubetis.92.fmsd,