Add support for isolation levels

[hab6]

The SQLAlchemy documentation on Transaction Isolation Levels states that SQLAlchemy dialects should support these isolation levels as well as autocommit to as great a degree as possible.

The following methods should be added to class IngresDialect:

• reset_isolation_level()
• set_isolation_level()
• get_isolation_level()
• get_default_isolation_level()
• get_isolation_level_values()
• .....

Reference: interfaces.py