unify prescribed soil types

[juliasloan25]

Is your feature request related to a problem? Please describe. Currently, we have two prescribed soil types: one for the canopy (PrescribedSoil) and one for soil co2 (SoilDrivers). It would be good if we can find a way to unify these types, and maybe make them analogous to our AtmosDrivenFluxBC type. Please link to any related issues/PRs.

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