Let `create` and `write` round monetary field values before sending them to the
database. Pass the values to be written to `field.convert_to_column`, so that
the currency can be retrieved from the values, and the value be rounded.
Simplify the API as much as possible, and delegate all the implementation
details to the models `ir.model`, `ir.model.fields`, `ir.model.constraint` and
`ir.model.relation`.
This removal has led to a necessary refactoring:
- make the setup of field attributes extensible;
- make the instantiation of custom fields extensible;
- delegate model and field reflection to `ir.model` and `ir.model.fields`;
- move the implementation of sparse and serialized fields to module `base_sparse_field`;
Christophe Simonis