use ClimaAtmos.ClimaAtmosSimulation directly

[juliasloan25]

We define our own ClimaAtmosSimulation type, which creates a ClimaAtmos.ClimaAtmosSimulation, then takes some information from it and repackages it as a ClimaCoupler ClimaAtmosSimulation. We do it like this so that we can type ClimaAtmosSimulation <: Interfacer.AtmosModelSimulation to use dispatch throughout the coupler (e.g. in calculating surface fluxes). It would be nice to just create the ClimaAtmos.ClimaAtmosSimulation and use that, if we can figure out a way around the dispatch issue.