Rename Land -> Strat and promote soil comps to state variables

This commit reverses two previous design changes:

1. Declaring variables on a Layer or Process only is again supported.
2. Soil contents are once again state variables.

These changes are intended to improve flexibility for future process implementations.

Additional changes:

• Renames Land module to Strat
• Removes defunct "partitioned enthalpy" scheme
• Renames z field in LayerState to bounds to allow for user-defined state variables named z.