dot: fix printing of alternating automata
Related to #208. * spot/twaalgos/dot.cc: Fix missing definitions of universal nodes, and inclusion of universal nodes inside of SCC when none of the destination comes back to the SCC. * tests/python/_altscc.ipynb: Adjust and add more test cases. * tests/core/alternating.test, tests/core/neverclaimread.test, tests/core/readsave.test, tests/core/sccdot.test, tests/python/decompose.ipynb: Adjust test cases. * NEWS: Mention the bug.