Implements is_streett_like() and streett_like_pairs(), is_rabin_like...

Adds the method spot::acc_cond::is_streett_like() that behaves like
spot::acc_cond::is_streett() except that it works on a wider range
of acceptance conditions, called Streett-like. Also adds
spot::acc_cond::streett_like_pairs() that returns a boolean assessing
whether the acceptance condition is Streett-like and also returns all
the Streett_like pairs.
Defines the new struct type spot::acc_cond::rs_pair.
Similarily, Adds the methods spot::acc_cond::is_rabin_like() and
spot::acc_cond::rabin_like_pairs().

* NEWS: Mention this modification
* python/spot/impl.i: Declares the new struct to SWIG, and defines
the streett_like_pairs() vector as an output parameter, which makes
the python code return a tuple (boolean, vector) rather than a
pass-by-reference vector.
* spot/twa/acc.cc, spot/twa/acc.hh: Declares an implements the new
methods and the new nested struct.
* tests/Makefile.am: Add new tests to the suite
* tests/python/rs_like.py: Tests the new methods and
the SWIG bindings.

tests/Makefile.am

@@ -354,6 +354,7 @@ TESTS_python = \
   python/sccinfo.py \
   python/setacc.py \
   python/setxor.py \
+  python/rs_like.py \
   python/sum.py \
   python/trival.py \
   python/twagraph.py \