Litter model should average abiotic quantities over relevant soil layers

[jacobcook1995]

Is your feature request related to a problem? Please describe.
We now have a system where the soil layers expected to be biologically active is defined within the layer structure, but currently the litter model doesn't use this it just takes abiotic variables from the first soil layer.

Describe the solution you'd like
Instead the litter model should average the abiotic variables over the relevant layers.

Describe alternatives you've considered
More complicated schemes of averaging could be used, i.e. weighting towards the surface layers as these are more active. But to begin with I think a simple average is enough