grazing type should be a subset of zooplankton type

[mnlevy1981]

Rather than have grazing(prey_cnt, pred_cnt), the zooplankton_type should have a grazer_prey_cnt variable and then an allocatable grazing(:) structure. Basically, this replaces grazing(prey_cnt, pred_cnt) with zooplankton(zoo_cnt)%grazing(:) (Per #317 these names will likely change).