Bank Balance Earns Interest V1.2

Adds interest calculations so that a farm's bank balance earns interest at the beginning of each month. The interest is displayed as a "Bank Interest" subtotal on the finances screen. The default rate is 1% APY. Two developer console commands have been added to manage the interest rate: `biSetInterestRate` – allows you to change the interest rate. Example: `biSetInterestRate 0.01` sets the rate to the default. `biGetInterestRate` – displays the current interest rate in the console. V1.2.0.0 Adds a "Bank Interest" money category.