postproc: add support for co-Büchi output
* spot/twaalgos/cobuchi.cc, spot/twaalgos/cobuchi.hh (to_nca): New function. (weak_to_cobuchi): New internal function, used in to_nca and to_dca when appropriate. * spot/twaalgos/postproc.cc, spot/twaalgos/postproc.hh: Implement the CoBuchi option. * python/spot/__init__.py: Support it in Python. * bin/common_post.cc: Add support for --buchi. * bin/autfilt.cc: Remove the --dca option. * tests/core/dca.test, tests/python/automata.ipynb: Adjust and add more tests. In particular, add more complex persistence and recurrence formulas to the list of dca.test. * tests/python/dca.test: Adjust and rename to... * tests/core/dca2.test: ... this. Add more tests, to the point that this is now failing, as described in issue #317. * tests/python/dca.py: Remove. * tests/Makefile.am: Adjust.