Fix issue with recipe ref fire dependency

[jlenh]

Description

• Closes Error in recipe REF fire when loading the trace file with arviz #4419
• Link to documentation:

This PR solves an issue arising in ref/recipe_ref_fire.yml because of an update in the dependency package arviz used to read in a netCDF file. Since the dependency now only uses its from_netcdf function as an alias to xarray.open_datatree, the dependency is being removed and the code updated to work with a xarray.DataTree object.

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[valeriupredoi]

thanks, Julien 🍺