Make it easier to "turn off" the canopy

[kmdeck]

Is your feature request related to a problem? Please describe. In global runs, we will have areas with and without a canopy, but due to software restrictions from using ClimaCore, every point in the domain requires a CanopyModel with the same number of prognostic variables.

Option 1. Run the same model everywhere (i.e. even when there is no canopy present, there will still be a canopy model with prognostic variables, but these will not change in time, and all canopy fluxes will be zero). In this case, make it easier to set up a default with no canopy present (LAI = RAI=0), i.e. dont require the user to set parameters, canopy height information, etc. Currently, PR #238 makes it so setting LAI=RAI=SAI gives zero transpiration, root extraction, absorption of radiation, and zero fluxes with the plant system, but you still need to define the parameters...

Option 2: Alternatively, if we run each column in the simulation independently, we may be able to set up something that has a different # of prognostic variables at each point. This feels more natural (though it wont work when we have lateral flow in the soil, since then each column is not independent; so this is not a long term solution, Option 1 or similar is unavoidable at this point). In this case, we would need to make it so there is a "no canopy" version of the model with no prognostic vars, parameters, etc. TBD if this type of setup would be harder for the coupler to work with for now as well.

Describe the solution you'd like Let's discuss