Impact testing plugin implementation

In Studio, the built-in implementation of this plugin is the ImpactTestingPlugin.gs class in the gw.plugin.rateflow package.

This plugin creates test periods by creating policy renewals. You can customize the implementation of this plugin directly. For example, you may want to create test periods in another way, such as by copying a submission or spinning off a policy. You can also change how the alternate rate books are selected.