* NEWS: mention the modification.
* python/spot/impl.i: makes to_weak_alternating visible from python
* spot/twaalgos/Makefile.am, spot/twaalgos/toweak.cc,
spot/twaalgos/toweak.hh: Implements to_weak_alternating.
* tests/Makefile.am, tests/python/toweak.py: Test the results of
to_weak_alternating.
