Bank Balance Earns Interest V1.2

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. 1.1.0.0 – Removes unnecessary global variables V1.1 fix: Eliminate global variable leaks in onPeriodChanged() by @XelaNull in #1