OPT_SPREAD_APPROX(call_put_flag,fut_price1,fut_price2,strike,time,rate,volatility1,volatility2,rho)
call_put_flag: 'c' for a call and 'p' for a put
fut_price1: price of the first futures contract
fut_price2: price of the second futures contract
strike: strike price
time: time to maturity in years
rate: annualized risk-free interest rate
volatility1: annualized volatility in price of the first underlying futures contract
volatility2: annualized volatility in price of the second underlying futures contract
rho: correlation between the two futures contracts
OPT_BS, OPT_BS_DELTA, OPT_BS_RHO, OPT_BS_THETA, OPT_BS_GAMMA.