This should will come handy to implement the convertion from LTL to
alternating automata, and to handle automata with multiple initial
states.
* spot/twaalgos/alternation.hh, spot/twaalgos/alternation.cc: New files.
* spot/twaalgos/Makefile.am: Add them.
* python/spot/impl.i: Add bindings.
* tests/python/alternating.py: Test states_and.