Consider type hierarchy refactor to simplify diagnostic methods

[kmdeck]

We have many shared diagnostics between SoilCanopyModel and the LandModel. This leads to shared methods, which then dispatch off of an argument being of type Union{SoilCanopyModel, LandModel}.

It might be good to update our type hierarchy with a new abstract type - something like AbstractSoilCanopyModel, where we would have both SoilCanopyModel <: AbstractSoilCanopyModel and LandModel <: AbstractSoilCanopyModel. Maybe even AbstractIntegratedModel or AbstractComplexModel, but these values are specific to having canopy + soil so we might want to be more specific in the naming.