This mixes the subset construction (for 1-state rejecting SCCs) and
the breakpoint construction (for larger rejecting SCCs).  The
algorithm should probably be rewritten in a cleaner and more efficient
way, but that should do for a first version.  It should be easy to
extend it to support Büchi acceptance (since the breakpoint
construction works for this) when we need it.

* spot/twaalgos/alternation.hh,
spot/twaalgos/alternation.cc (remove_alternation): New function.
* tests/python/alternation.ipynb: New file.
* tests/Makefile.am, doc/org/tut.org: Add it.