P_CaCO3%prod(k) and P_SiO2%prod(k) were not being computed correctly in cases
where there were multiple calcifiers and silicifiers, respectively. POP%prod(k)
was missing the zoo_graze_poc term, which is non-zero when zooplankton grazes
on non-zooplankton.

The default CESM configuration only has one calcifier and one silicifier, and
the single zooplankton does not graze on itself, so this should not be answer
changing when PFT_defaults = "CESM2".