I'm trying to create a function to allow me to calculate total interest paid
on a loan.

* The interest is calculated daily (using "annual rate/365" to calculate
daily rate)
* Accrued interest is applied to the loan monthly on the last day of the
calender month.
* Payments will be regular, either fortnightly or monthly.

This will be a generic function, but needs to be accurate as it may be
potentially used for large amounts / long periods. I can't work out how to do
this without filling a sheet with daily calculations.