A sigmoid shaped selectivity function. Based on two parameters
l50 which determine the length at which 25% and 50% of the stock is
selected respectively. As the size-based model is weight based, and this
selectivity function is length based, it uses the
b to convert between length and weight.
sigmoid_length(w, l25, l50, species_params, ...)
the size of the individual.
the length which gives a selectivity of 25%.
the length which gives a selectivity of 50%.
A list with the species params for the current species.
Used to get at the length-weight parameters