call_put_flag: 'c' for a call and 'p' for a put
spot: spot price
strike: strike price
time: number of days to exercise
domestic_rate: domestic risk-free interest rate to the exercise date in percent
foreign_rate: foreign risk-free interest rate to the exercise date in percent
volatility: annualized volatility of the asset in percent for the period through to the exercise date
OPT_GARMAN_KOHLHAGEN values the theoretical price of a European currency option struck at strike on an asset with spot price spot.