Price European spread options using Kirk pricing model

returns the price for a European spread option using the Kirk pricing model.`Price`

= spreadbykirk(`RateSpec`

,`StockSpec1`

,`StockSpec2`

,`Settle`

,`Maturity`

,`OptSpec`

,`Strike`

,`Corr`

)

[1] Carmona, R., Durrleman, V. “Pricing and Hedging Spread Options.”
*SIAM Review.* Vol. 45, No. 4, pp. 627–685, Society for Industrial and
Applied Mathematics, 2003.