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.
