* spot/twaalgos/rabin2parity.cc, spot/twaalgos/rabin2parity.hh:
implement it
* spot/twaalgos/postproc.cc: use it
* spot/twaalgos/Makefile.am: build the new files
* NEWS: document the new function
* python/spot/impl.i: Python bindings for the new function
* tests/Makefile.am, tests/core/rabin2parity.test: test the new function
