Grow a mean length forward by a fraction of a year
grow_increment.RdThe increment of the Richards (or von Bertalanffy, rho = 1) curve
over an elapsed time e from a current mean length L:
$$L(t + e)^\rho = L_\infty^\rho + (L^\rho - L_\infty^\rho) e^{-K e}$$
Applied to a length that sits on the curve it returns the curve's value
e later, so splitting a year into seasons changes nothing; applied to a
length kept from an earlier year's parameters it is how a cohort keeps its
own history.