Add support to produce output locally for `model-levels-to-zarr.py` and `single-levels-to-zarr.py`

[dabhicusp]

I am facing challenges while attempting to generate output locally of the model-levels-to-zarr.py and single-levels-to-zarr.py scripts. I have reviewed the provided documentation and source code, but I am still uncertain about the necessary steps to execute these scripts and produce the desired output files locally.