Connect excrement and carcass flows to the soil model

[jacobcook1995]

Is your feature request related to a problem? Please describe.
In PR #562 six new variables are added to track carbon, nitrogen and phosphorus flows into the soil due to decayed carcasses and excrement. These flows do not connect to anything in the soil model.

Describe the solution you'd like
These flows need to be connected to the relevant pools in the soil model.