In Model converter, also validate messages in ``message_to_odoo``.

Model converter argument changed, `__type__` is now the last argument and is optional. It is expected not to be used
anymore.
Added possible_datatypes property and odoo_datatype on Converter. Defaults to empty set and None.
On Model, it can be set.

Replace generic exceptions.

 Use ©

In the process:
* Update based on latest xcg/templates/odoo_module.
* Fix existing MailTemplate converter (found by these new tests).

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.

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.