alternation: implement remove_alternation() for weak alt automata
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.