Version controlling schema

[PrasadBabarendaGamage]

1. See https://sqlalchemy-migrate.readthedocs.io/en/latest/versioning.html#making-schema-changes for how to create a python schema change script
2. Inside the python schema change script load SDS 2.0 schema from the schema creation github issue as a sqlalchemy Table
3. Add upgrade/downgrade migrate functions that will add a new dummy field to SDS 2.0 data_set_description
4. Apply schema change script create/export a new SDS 2.1-alpha schema
5. Create example
6. Create first part of tutorial 2