We convert all the fields that were storing python code into PYSON.
We add a pyson attribute on field tag to store PYSON value.
issue4416
review6971002
Only active_mode, active_id and active_ids can be used on relate domain because
it is the only values which could be sur to be present.
Also it always try to use active_ids over active_id to give more possibilities
to the user.
issue2474
review3261002
Cédric Krier