Add function to library that allows selectivity = maturity calculations to define SSS selectivity inputs

[shcaba]

slope<- -0.5683
intercept<-21.84
x<-seq(1,50,0.05)
mat.vec<-1/(1+exp(slope*(x-intercept)))
mat.vec[mat.vec>0.95]
width.95per<-(x[mat.vec>0.95]-intercept)[1]
width.95per