Plant mortality should add litter and fruit to litter pools

At the moment `PlantsModel.apply_mortality` only adds the stem mass to the deadwood pool. It should also add the other masses (stem, root, reproductive tissue) to appropriate litter pools.

Start by locating PlantsModel.apply_mortality and inspect how stem mass is currently added to the deadwood pool. Identify the appropriate litter pools for root and reproductive tissue, then verify that mortality adds each mass to the intended pool.

Written by the indexing model from the issue text.