* Start work on breaking down some of the Financial Excel functions
* Unhappy path unit tests for Treasury Bill functions
* Codebase for Treasury Bills includes logic for a different days between settlement and maturity calculation for OpenOffice; but Open/Libre Office now uses the Excel days calculation, so this discrepancy between packages is no longer required
* We've already converted the Settlement and Maturity dates to Excel timestamps, so there's no need to try doing it again when calculating the days between Settlement and Maturity
* Add Unit Tests for the Days per Year helper function
* Extract Interest Rate functions - EFFECT() and NOMINAL() - with additional validation, and unhappy path unit tests
* First pass at extracting the Coupon Excel functions
* Simplify the validation methods
* Extended unit tests to cover all combinations of frequency and basis, including leap years
Fix for COUPDAYSNC() when basis is US 360 and settlement date is the last day of the month
* Ensure that all Financial function code uses the new Helpers class for Days Per Year
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