Pass restart file via initial condition argument

[nefrathenrici]

With the AtmosSimulation constructor, it does not really make sense to be able to pass both an initial condition and a restart file. We should enable passing a restart file via the initial condition instead.

[haakon-e]

One potential way to pass a restart file through the initial_condition kwarg, is

@kwdef struct RestartFile
    path::String
end

AtmosSimulation(;
    initial_condition = RestartFile(; path = "path/to/output_0001/day1.0.hdf5"),
    # ...
)

which replaces the restart_file kwarg.

To accommodate the detect_restart_file kwarg, you could do e.g.

struct DetectRestartFile end

AtmosSimulation(;
    initial_condition = DetectRestartFile(),
    # ...
)

Then setup_output_dir, which currently handles restart_file and detect_restart_file could e.g. dispatch on ::RestartFile and ::DetectRestartFile to handle the associated logic.

---

A notable difference between restart files and "regular" initial conditions, is that, currently, restart files dictate the model grid. At a minimum, we should probably notify a user if the provided grid differs from that in the restart file.