age within stage models

[levisc8]

Better thought of as an extension of age_x_size (I think). Basically need a function called something like define_age()/define_age_within_stage() with an interface like:

define_age(age = list(yearling = c(0,0),  
                                   small = c(1,4),
                                   large = c(5, 10), 
                                   greybeard = c(max_age, max_age))

This info then gets folded into .init_iteration.