Function that returns a stochastic estimate of steepness given observed stock recruitment data
SRopt(nsim, SSB, rec, SSBpR, plot = F, type = "BH")number of samples of steepness to generate
'observations' of spawning biomass
'observations' (model predictions) of recruitment
spawning stock biomass per recruit at unfished conditions
should plots of model fit be produced?
what type of stock recruitment curve is being fitted BH = Beverton-Holt