Move Soil to Physics module and rename Water -> WaterBalance
Minor internal refactoring which moves Soil and all related soil processes to a new Soils submodule of Physics. This seems to make more sense than having a separate Layers module and cleans up the dependency structure of the physics modules; i.e. HeatConduction and WaterBalance should be independent of each other, and coupling functionality should exist in other modules such as Soils.
