split: add separate_edges() and a edge_separator class

This generalizes (and replaces) the two-argument split that was
introduced in c2832cabfc.

* spot/twaalgos/split.cc, spot/twaalgos/split.hh (edge_separator): New
class.
(separate_edges): New function.
(split_edges): Remove the two argument version.
* spot/twaalgos/forq_contains.cc: Adjust to use the edge_separator
class.
* tests/python/splitedge.py: Adjust test case.
* tests/python/split.ipynb: New file.
* tests/Makefile.am, doc/org/tut.org: Add it.
* NEWS: Mention it.

tests/Makefile.am

@@ -378,6 +378,7 @@ TESTS_ipython = \
   python/randaut.ipynb \
   python/randltl.ipynb \
   python/satmin.ipynb \
+  python/split.ipynb \
   python/stutter-inv.ipynb \
   python/synthesis.ipynb \
   python/testingaut.ipynb \