Issue 172

[tjduigna]

Closes #172

Provides the RawDAO and DAO classes as a relatively simple interface for fetching and persisting data in relational data storage systems. Additionally provides a number of pytest fixtures for sqlite and postgres and test examples that demonstrate the API.

Thoughts so far:

• sqlalchemy and psycopg2 can be optional dependencies
• the CI build should install optional dependencies and execute all tests
• we can inform the user attempting to use the DAO API if dependencies are not installed? or is throwing the ImportError sufficient

[tjduigna]

Opting to remove the related functionality as it was not well thought out and fundamentally limited in its functionality. It should be redesigned to support generic join statements.

I think better support for SQL clauses that come after the FROM like GROUP BY, ORDER BY, and HAVING is worth adding here. This way the implementation of the RawDAO is complete and single-table only. Extending the DAO for related queries and offloading that work to the database can be an enhancement on the single table API.