Remove concatenation of zarrs from post processing scripts

[nbren12]

We currently concatenate zarrs when running runfv3 append into the <rundir>/name.zarr. This is convenient for analysis but has several downsides:

• The concatenation code can and does corrupt the zarr files when the post-processing is interrupted.
• complexity: gsutil-based implementation of appending to a zarr is complex. This constrains the chunk sizes we can use, which then leads to chunk size validation etc.

There are a couple ways to pay down the debt:

• reimplement appending with ffspec/normal zarr operations now that gcsfs is more reliable
• simply save zarrs into each artifacts// directly and add some functions for opening data

[oliverwm1]

Not convinced we want to introduce the dependency, but could use https://github.com/fsspec/kerchunk to enable opening multiple zarr's easily.