Feature Request: discount rates that change over time

I'm doing some economic analyses over 100 years, and in the UK the [Green Book](https://www.gov.uk/government/publications/the-green-book-appraisal-and-evaluation-in-central-governent) has discount rates that change at a couple of break points.

Right now I'm running the model without discounting, then running get_values to discount them manually. 