Price European spread options using Bjerksund-Stensland pricing model

`Price = spreadbybjs(RateSpec,StockSpec1,StockSpec2,Settle,Maturity,OptSpec,Strike,Corr)`

returns the price for a European spread option using the Bjerksund-Stensland pricing
model.`Price`

= spreadbybjs(`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.

[2] Bjerksund, Petter, Stensland, Gunnar. *“Closed form spread option
valuation.”* Department of Finance, NHH, 2006.