Fixes#255.
* spot/twaalgos/split.cc, spot/twaalgos/split.hh,
tests/core/split.test: New files.
* spot/twaalgos/Makefile.am, tests/Makefile.am: Add them.
* bin/autfilt.cc (--split-edges): New option.
* python/spot/impl.i: Process split.hh.
* tests/python/alternating.py: Test split_edges() on
an alternating automaton.
