Implement an Interest Rate Timeline for the penalty rate

[bosue]

The penalty interest rate is usually governed by the Central Bank reference rate, so may change frequently while not necessarily being an arbitrary business decision.

For periods before the reference rate changed, the old interest rate needs to be used.

Take as an example, an unpaid sales invoice or loan installment due 20.06.2023.
In Germany the late payment interest rate is legally defined as 8 p.p. above the Bundesbank reference rate which itself is derived from the ECB reference rate. Effectively we‘re supposed to bill:

10.62 % for 10 days until 30.06.2023 = EUR 3.12
12.12 % for 62 days starting 01.07.2023 = EUR 10.50
= late payment interest: EUR 13.62

I don‘t have to detail how this would look like for a bill due since last year or an even older Receivable. Reference rates may change several times and still need to be correctly reflected in today’s dunnings, period by period.

To reflect this, we need a flexible and reliable mechanism that may then be used throughout ERPNext, both for standard dunnings and for our penalty interest rates here.

In frappe/erpnext#37095, I therefore proposed an Interest Rate Timeline DocType in the ERPNext queue that may then be perused by us here as well.

Next step, even a Variable-rate Loan‘s main Rate of Interest may be derived from this, but let‘s start with the lower-hanging fruit.