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.

tests/Makefile.am

@@ -309,11 +309,15 @@ TESTS_twa = \
   core/cycles.test \
   core/acc_word.test \
   core/dca.test \
+  core/dca2.test \
   core/dnfstreett.test \
   core/parity.test \
   core/parity2.test \
   core/ltlsynt.test
 
+# Issue #317.
+XFAIL_TESTS = core/dca2.test
+
 ############################## PYTHON ##############################
 
 if USE_PYTHON
@@ -356,7 +360,6 @@ TESTS_python = \
   python/bdditer.py \
   python/bddnqueen.py \
   python/bugdet.py \
-  python/dca.test \
   python/declenv.py \
   python/decompose_scc.py \
   python/dualize.py \
@@ -410,7 +413,6 @@ nb-html: $(TESTS_ipython:.ipynb=.html)
 EXTRA_DIST = \
   $(TESTS) \
   python/ltl2tgba.py \
-  python/dca.py \
   python/ipnbdoctest.py