eTraGo works with ego.io 0.3.0

eTraGo was updated to the latest ego.io version. Therefore it uses the new sessionmaker for database connections without using oemof.db.
Moreover the result-to-oedb interface was improved. It is now possible to state that a result set shall be versioned in the schema 'grid'.
Some more minor features were introduced like the skip_snapshots argument which easily enables to skip snapshots in order to naively simplify the optimization problem.