Improve leaf-litter decomposition by limiting to microbial activity (not macroinvertebrates)

The dataset used for parameterisation (in PR #70) mixed fine- and coarse-mesh litter bags, so the decay rates are actually a mixed result from microbial (fine) and macrofaunal (coarse) activities. It is not crucial to fix this for now, but we can simply improve it by filtering the dataset to only use fine litter bags. 