Fix bug with parameterized heterogeneous soil layers

Additionally refactors soil parameterization again...

• Removed Nonparametric; Nothing is used to indicate nonparametric configuration.
• Add SoilParameterization abstract type.
• Remove ByComposition and replace ByXicePorSat with simply BySoilProperties. This parameterization is favored anyway because 1) there are no cross-parameter constraints, and 2) less intrinsic correlation between parameters.