Integrate Unitful.jl units for forestry calcs

[Crghilardi]

This is more of a long-term goal, but the ability to define logic for metric and imperial calculations is a huge opportunity and I think would be massively popular. Currently, most models I see are written in one or the other with no "easy" way to switch or select. See this ticket for the FVS example where they have to chose when and where to convert and other difficult decisions.