Code for natural-area SRFC Smsy based on updated Munsch et al fry index

Version 2 updated to include calculation of confidence intervals on S_mp via likelihood profiling on beta. Code for estimation of a natural-area Smsy for Sacramento River Fall Chinook, using an updated version of the Munsch et al. (2020) fry index as the metric of recruits and the relationship (for a Ricker applied to salmon) Smsy = Fmsy * Smp where Smp is the escapement that maximizes recruitment, 1/beta.