Add Multi-year/Multi-rate Billing

[thomastu]

Rates change over time - we need some abstraction to describe using a schedule for rate schedules for timeseries that span across multiple rates. Two mechanisms would motivate this behavior:

1. A customer changes their rate (potentially mid-year)
2. A PUC or ESP changes their rates year on year.

A basic conceptual model of this feature would involve specifying a Calculator object (which is tied to a rate) with a valid date range.