External estimation procedure for von Bertalanffy growth.
vbgf_func(L1, L.inf, k, ages, a3)mean length at youngest age which is well sampled in the data (a3)
Length at infinity
von Bertalanffy growth rate parameter
vector of ages in the data for which you want to predict mean length-at-age
youngest age which is well sampled in the data
a vector of lengths predicted which correspond to the input ages vector.