Bank Balance Earns Interest

Adds interest calculations so that a farm's bank balance earns interest at the beginning of every month. The interest is included in the "miscellaneous" subtotal on the finances screen. The default rate is 1% APY.

Two dev console commands have been added to control the interest rate:biSetInterestRate - allows the modification of the interest rate. ex: biSetInterestRate 0.01 would set the interest rate to the default.biGetInterestRate - prints the current interest rate to the console.