Reduce duplication in statebc methods

[kmdeck]

Describe the solution you'd like All of our state BC's require extracting levels of fields and returning a finite difference estimate of a gradient multiplied by another level of a field. Instead of having 6 methods (2 for water, heat, carbon), let's try and combine these.

We can also improve the dispatch used for water and heat in the soil model, for state BC, since this relies on # of arguments instead of type. This issue is compounded for every model that adds in their own StateBC method.