Fix issue with Switch and the use of Skip as a converter, also make switch send Skip when nothing matches its rules.

Fix issue with RelationToMany that always convert to Skip when send_empty is False.

Fix issue with chaining relation converter with empty values.

Change get__type__ to return a set of values.
It is defined at the Converter level despite only returning a value for a small number of Converters but it simplifies the code using it to do so that way.

Validation and validator can be set on converters after initialization. Only some converter makes use of those values.
This makes the message_to_odoo_validate method unnecessary, that’s why it has been removed.

 avoid legacy converters in tests, add warnings on deprecation, include the functionnality of FirstKeyField in KeyField

See ``tests/test_relation.py > test_many2one_to_odoo`` for a test case.

``get_instance`` functions can now return a ``Newinstance`` record (record being created).

 Add tests, correct code so test passes.
Fix issues with some converter.

add jsonschema requirement

Send datetime value to redner with timezone.

Split converters in multiple modules
Remove deprecated converters
Move legacy converters in a dedicated module