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.

GitHub: https://github.com/EvanKirsch/fs25_bankAccountInterest