Animals are not 100% carbon

**Is your feature request related to a problem? Please describe.**
At the moment the animal model only tracks total animal mass. This is the standard for calculating size scaling relationships. However, when thinking about carcass decay we mainly care about the fraction of the mass that is carbon, this "carbon mass" needs to be tracked

**Describe the solution you'd like**
At some point this tracking should be added.

At present the litter model uses the total mass, where it really should be using the "carbon mass". So, it needs to be borne in mind that the estimates of inputs to the soil from the `animal` will at present be a significant over estimate.