compile a list of requirements for coupling component models

[LenkaNovak]

We have a unified interface for coupling component models, which needs to be formalized in the documentation. We would also like to require certain standalone run benchmarks:

• demonstrating a global simulation with other components prescribed

• global conservation checks

• performance benchmark in the coupler

• Coupler variables:

- `T_S` = surface temperature [K], req. for
    - radiative flux calculation (atmosphere), turbulent flux calculation (atmosphere or coupler)
- `z0m_S` and `z0b_S`= roughness length for momentum and buoyancy [m], req. for 
    - turbulent flux calculation (surface -> atmosphere or coupler)
- `ρ_sfc` = [], req. for
    - turbulent flux calculation (surface + atmos -> atmosphere or coupler)
- `q_sfc` = [], req. for
    - turbulent flux calculation (surface + atmos (ρ_sfc) -> atmosphere or coupler)
- `albedo` = [], req. for
    - radiative flux calculation (surface -> atmosphere)
- `beta` = [], req. for
    - turbulent flux calculation (surface -> atmosphere or coupler)
- `F_turb_energy` = sensible + latent heat fluxes consistent with our total energy formulation [], req. for
    - boundary conditions for component models (atmosphere or coupler -> component models)
- `F_turb_moisture` = evaporation [], req. for
    - boundary conditions for component models (atmosphere or coupler -> component models)
- `F_turb_ρτxz` and `F_turb_ρτyz` = drag in the zonal and meridional directions [], req. for
    - boundary conditions for component models (atmosphere or coupler -> component models)
- `F_radiative` = surface radiation fluxes [W/m^2], req. for
    - boundary conditions for component models (atmosphere -> component models)
- `P_liq` and `P_snow` = precipitation of water and snow [], req. for
    - surface accumulation, conservation of water (atmosphere -> surface models)
- `F_radiative_TOA` = top-of-amtosphere radiative fluxes [W/m^2], req. for
    - diagnostics (top of atmosphere balance)
- `P_net` = [], req. for
    - 

[LenkaNovak]

The complete list is in this issue which will implement compatibility tests in dependent component models. Standalone benchmarks are implemented in #355