* spot/twaalgos/parity.cc: cleanup_parity and cleanup_parity_here are
now better at finding useless parity colors
* tests/python/parity.py: test it
* NEWS: document the change
* spot/twaalgos/parity.cc: Do not throw a pointer to an exception,
throw the exception directly. Factor all the throwing code in a
function.
* tests/python/parity.py: Add test case.