Consistent treatment of energy across models

**Is your feature request related to a problem? Please describe.**
At the moment the `animal` model tracks energy directly, while the `soil` and `litter` models track carbon, which is an imperfect proxy for energy. This becomes a problem when exchanges between models take place, as conversions have to be made. At the moment the one example of this conversion (animal excrement to metabolic litter) is done in a very handwavey manner. This is something we need to fix for more complete versions of the Virtual Rainforest.

**Describe the solution you'd like**
The fact that different models track different quantities is unavoidable as they are drawn from different traditions with different objects of study. We could enforce all models tracking exactly the same quantities, but this doesn't allow us to escape the problem as we would still need to find sensible conversions between quantities.

I think we need to decide on a consistent approach to conversions of this type, be it making a particular set of assumptions, literature search or something else. I feel the key thing to minimise the problem is that we take a consistent approach to these conversions both within and across models.

**Describe alternatives you've considered**
I don't feel there is any quick way to fix this, so feel that eventually having an extended discussion of the problem will be necessary at some point in the future.

Start by reviewing the animal, soil, and litter models and the existing conversion from animal excrement to metabolic litter. Determine which assumptions or literature should govern conversions between energy and carbon, then document an agreed consistent approach for exchanges within and across models.

Written by the indexing model from the issue text.