Generate PostgreSQL DDL from WMDR UML model

Generate PostgreSQL DDL from UML model (see [WMDR#40](https://github.com/wmo-im/wmdr/issues/40)) including finding a way to create the WMDS code tables in the generated SQL DDL (or as part of the SQLAlchemy ORM model [using choices](https://sqlalchemy-utils.readthedocs.io/en/latest/data_types.html#module-sqlalchemy_utils.types.choice))