Added Writeonly converter. (features & fixes)

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

field.py

@@ -133,7 +133,7 @@
         # do not include value if already the same
         if instance and instance is not Newinstance:
             value = self.odoo_to_message(instance)
-            if value is Skip or value == message_value:
+            if isinstance(value, SkipType) or value == message_value:
                 return {}
         if self._odoo_formatter:
             message_value = self._odoo_formatter(message_value)