Calculate selectivity curve

## Usage

`getsel(x, lens, lfs, sls, srs)`

## Arguments

- x
Simulation number

- lens
Matrix of lengths (nsim by nlengths)

- lfs
Vector of length at full selection (nsim long)

- sls
Vector of sigmas of ascending limb (nsim long)

- srs
Vector of sigmas of descending limb (nsim long)