Add documentation and possibly utilities for database migration in tests for clients of pycds

[rod-glover]

Scenario: Your project depends on pycds. You want to test it.

Problem: You can no longer just use pycds.Base.metadata.create_all(): that creates only the tables, and pycds now includes non-table items such as views, native materialized views, and functions. The only way to get those is to migrate an empty database to the current revision. This calls for some knowledge which is not immediately available to the innocent user. For example, see the test setups for

• station-data-portal-backend
• pydap-extras

Solution:

• Add documentation around this
• Export some handy fixtures