API definitions - intervals vs n-payments

[alecloudenback]

In trying to extend the built-in functions to include immediate annuities, I have encountered an area that highlights an ill-defined aspect of the API:

Currently, ä(ins,5) (ä(ins,to_time)) refers to the annuity-due payments that occur between times [0,5), so that includes five payments.

In extending this to an annuity certain, a(ins,5) referring to the interval [0,5) would only receive four payments at times 1,2,3,4.

This is inconsistent with how the literature portrays a(ins,5) as the 5 refers not to time but number of payments.

Question is: change the API to be consistent with the literature?

[alecloudenback]

relevant PR: #31

[alecloudenback]

I think being consistent with the literature is more important than a modeling-oriented approach like the timesteps. Will refactor to focus more on the maths and less on the modeling here and then continue the modeling work in another package.

[alecloudenback]

31