enable pv and sv annuity methods to handle general accumulation functions

[genedan]

Right now the Annuity class isn't well-equipped to handle general accumulation functions. See Vaaler & Daniel textbook example 3.3.12 where the interest rate changes from .06 to .05. The shortcut formulas are only valid when the compound interest rate does not change.

[genedan]

There's a section further in the textbook that mentions general accumulation function, so we can wait until we reach that point.