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.

assigned to @bgroenke

@mlanger This should (mostly) resolve the redundancy in declaring variables in new layers/processes.

One can now simply define:

variables(::Heat) = (...)
variables(::Soil) = (...)

which are by default combined in:

variables(layer::Layer, proc::Process) = (variables(layer)..., variables(proc)...)

So for snow, one could, in principle, add the additional variables in variables(::Snow) without having to redefine the variables defined by variables(::Heat). In the event that a specific Layer/Process pair needs special treatment, one can still explicitly define:

variables(::Snow, ::Heat) = (...)

which can then call variables for either (or both) of the layer or process individually to re-use variable definitions, or just redefine everything if changes are necessary.

Tests pass locally.

approved this merge request

merged

mentioned in commit 9ea13637