FI 24 48: Really - Compute Interest!

* Refactor: Make scenario required for value() in Leia and Amount
* Let Amount compute interest given the interest rate and the start month
* Make growth (interest) work!
* Calculate monthly rate as 12th root of (1 + annual %)
* Calculate per-month by taking starting value * monthly-rate ^ #months
* See smooth growth curve on a simple annuity