Nature submodel

There are two source files in the src/nature directory that are currently not used: insects.jl and energy.jl. The first defines a function insectbiomass() that provides a very rough estimate of insect population density in a given pixel. The second is a (still incomplete) implementation of Dynamic Energy Budgets. Both were begun in the expectation that they would be needed, but then set aside for the time being. We note their existence here should they become useful again (with the caveat that both require testing).