- Rename `Model.readonly` to `Model._readonly` to prevent collision with a column that has the same name
- The `field_names` argument to `Model.to_tsv` is now mandatory
1) Divided readonly and system flags of Field model. Readonly flag only restricts insert operations, while system flag restricts also create and drop table operations
2) Added Merge engine and tests for it
3) Added docs for Merge engine
4) Added opportunity to make Field readonly. This is useful for "virtual" columns (https://clickhouse.yandex/docs/en/single/index.html#virtual-columns)
