Refactors the configuration of freeze curves in heat conduction and soil types

Since the removal of the old SFCC type in FreezeCurves which included a nonlinear solver with the freeze curve, the solver is specified in the Soil type. This doesn't really make sense, however, since the solver is not directly related to the soil itself and is rather a numerical detail. Furthermore, it is conceivable that other types of materials could potentially also induce implicit constitutive relations that would require such a solver for the inverse enthalpy formulation. The solver is therefore now specified in the operator type when necessary. No solver need be specified for heat conduction formulations that do not require it, e.g. the apparent heat capacity formulation (now referred to as ForwardTemperature in the code).