Create central registry for state variable definitions

[bgroenks96]

One of the drawbacks of the CryoGrid.jl interface for modular specification of state variables, i.e:

variables(::SoilEnergyBalance) = (
    prognostic(:temperature,  XYZ(), units=u"°C", desc="Temperature of the grid cell in °C"),
    ...
)

is that it relies on the developer "just knowing" the state variable naming conventions and results in possible redundant or conflicting metadata (i.e. units, description, bounds, etc.) when implementing multiple processes across different models or components.

We could ameliorate this by creating a kind of state variable "registry" where the abstract variables are defined, and then only specify them as prognostic, auxiliary, etc. in the variables method for each component. I think SINDBAD does something like this, although there the variable registry seems to be primarily a UX feature and is not used in the actual model/process definitions...? @dr-ko can maybe clarify.

Anyway, we would add something like:

const SoilVars = map(to_namedtuple, (
    statevar(:temperature, units=u"°C", desc="Temperature of the grid cell in °C"),
    statevar(:saturation_water_ice, bounds=0..1, desc="Saturation level of water/ice in the soil pore space"),
    ...
))

and then implement variables as:

variables(process::SoilEnergyBalance) = (
    prognostic(SoilVars.temperature)
)

@maximilian-gelbrecht and @mahabadri what do you think?

[dr-ko]

That is correct, @bgroenks96 . We have a list of variables mostly for metadata and we do not have a central registry that is used for definition.
Given the stricter need in Terarium for the distinction of state variables, I agree with your idea of centralizing the definitions.

Perhaps, even better to build on top of CMIP standards if possible, as that is a natural target in the mid-term (?) I was looking at https://github.com/dr-ko/cmip6-cmor-tables/tree/master at the beginning for SINDBAD.jl, but decided to stick with our old/local conventions.