* spot/twa/acc.hh: Introduce parity_min_odd(n) and friends.
* spot/twaalgos/determinize.cc, spot/twaalgos/rabin2parity.cc,
spot/twaalgos/toparity.cc: Use them.
* tests/python/parity.py: Call each function exhaustively.
* NEWS: Mention the new functions.
* 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.