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.

 Add some typing information, or make it consistent.
 Add more docstrings.
 Fix using Skip in switch converter.

 Use ©

Used by other modules; these exports got removed when I replaced an ``import *`` earlier.

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.

update to use latest template, using | instead of Union as allowed by python 3.10 as it is more readable

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.