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 ©

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

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.

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.

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