for detailed description and inference of Geometric Asian Option, please refer to https://github.com/songqsh/MA6628v02/blob/master/pdf/L04supp01.pdf
- define a function to calculate the value of d1 which is a paramter of Black and Scholes model(BSM)
- define a function to calculate the call option value of BSM
- calculate the parameters of GAC such as sigma_hat and r_hat
- calculate the value of GAC by BSM evaluation