Use xarray-beam to append derived Replay variables

[timothyas]

This adds two python scripts that allow one to append derived variables to the smaller 1 degree and subsampled 1/4 degree Replay datasets.

Example usage can be found in submit_geopotential_append.sh, and it's very similar for the static variables.

Scaling up to append variables to the 1/4 degree datasets is future work, note that I couldn't even append the static variables to the 1/4 degree dataset. It seems like the steps of just opening the dataset and building the graph associated with appending to it already takes too much memory for a c2 standard 60 node.

The localzarr.py file has some modifications to the xarray_beam.zarr module necessary for appending variables. I'd like to add this to that code base if the developers are interested, but I thought this would be a good place to start.