streamlined PrescribedAtmos constructor

[juliasloan25]

Is your feature request related to a problem? Please describe.
Following CliMA/ClimaUtilities.jl#105, we're able to do transformations on multiple data variables to create our forcing variables (and can read them all from one file). It would be nice to use this to make a PresribedAtmos constructor that avoids the user having to do this:

  u_atmos = TimeVaryingInput(
        joinpath(era5_artifact_path, "era5_2021_0.9x1.25_clima.nc"),
        "ws",
        surface_space;
        reference_date = ref_time,
        t_start,
        regridder_type,
    )
    q_atmos = TimeVaryingInput(
        joinpath(era5_artifact_path, "era5_2021_0.9x1.25_clima.nc"),
        "q",
        surface_space;
        reference_date = ref_time,
        t_start,
        regridder_type,
    )
    P_atmos = TimeVaryingInput(
        joinpath(era5_artifact_path, "era5_2021_0.9x1.25.nc"),
        "sp",
        surface_space;
        reference_date = ref_time,
        t_start,
        regridder_type,
    )

etc for each variable and then

    atmos = PrescribedAtmosphere(
        precip,
        snow_precip,
        T_atmos,
        u_atmos,
        q_atmos,
        P_atmos,
        ref_time,
        h_atmos,
        earth_param_set,
    )

and instead just takes the file, earth_param_set, ref_time... etc and makes atmos directly. We may still have to create an object for each individual variable internally.

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