Refactor waq_scenario #4
Description
waq_scenario is poorly organized, encompassing too many different pieces. This is apparent with the problems of circular imports.
It is probably not the right interface for defining scenarios, either. Once a reasonable interface for dflow_model emerges, probably all of waq_scenario will get refactored to feel more like dflow_model. In general, this means that rather than customizing a scenario by subclassing, a scenario is an object which may be operated on, modified, etc. to build up a scenario. As needed, methods can be overwritten, and in some cases a subclass may be the easiest, but avoid defaulting to subclasses as the means of customization.