Introduce new ways to split an automaton

The explicit way of splitting suffers if there are
too many input APs, two new ways of splitting
are introduced as well as a heuristic to chose
between them.

* NEWS: update
* spot/twaalgos/synthesis.cc,
spot/twaalgos/synthesis.hh: New fonctions
* bin/ltlsynt.cc: Add corresponding option
* tests/core/gamehoa.test,
tests/core/ltlsynt.test,
tests/python/_partitioned_relabel.ipynb,
tests/python/_synthesis.ipynb,
tests/python/game.py,
tests/python/split.py,
tests/python/synthesis.py: Adjusting and adding test

NEWS

@@ -86,6 +86,15 @@ New in spot 2.12  (2024-05-16)
 
   Library:
 
+  - split_2step has now multiple ways to split for improved performance.
+    The option can be controlled via the synthesis_info::splittype
+    - EXPL: explicit splitting of each state as before
+    - SEMISYM: The outgoing transition of each state are encoded
+      as a bdd; Works better for larger number of input APs
+    - FULLYSYM: The automaton is first translated into a 
+      fully symbolic version, then split.
+    - AUTO: Let the heuristic decide what to do.
+
   - The following new trivial simplifications have been implemented for SEREs:
     - f|[+] = [+] if f rejects [*0]
     - f|[*] = [*] if f accepts [*0]