getBiomass: Calculate the total biomass of each species within a size range at each time
step.
Description
Calculates the total biomass through time within user defined size limits.
The default option is to use the whole size range. You can specify minimum
and maximum weight or length range for the species. Lengths take precedence
over weights (i.e. if both min_l and min_w are supplied, only min_l will be
used).