Work around xarray StringDType serialization error

[spencerkclark]

The evaluator integration test on beaker failed with the following error:

Details

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/opt/conda/lib/python3.11/site-packages/fme/ace/evaluator.py", line 9, 
in <module>
    main(args.yaml_config, override_dotlist=args.override)
  File "/opt/conda/lib/python3.11/site-packages/fme/ace/inference/evaluator.py",
line 289, in main
    return run_evaluator_from_config(config)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/fme/ace/inference/evaluator.py",
line 459, in run_evaluator_from_config
    aggregator.flush_diagnostics()
  File "/opt/conda/lib/python3.11/site-packages/torch/utils/_contextlib.py", 
line 116, in decorate_context
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File 
"/opt/conda/lib/python3.11/site-packages/fme/ace/aggregator/inference/main.py", 
line 573, in flush_diagnostics
    write_reduced_diagnostics(
  File "/opt/conda/lib/python3.11/site-packages/fme/core/diagnostics.py", line 
60, in write_reduced_diagnostics
    to_netcdf_via_inter_filesystem_copy(ds, filename)
  File "/opt/conda/lib/python3.11/site-packages/fme/core/cloud.py", line 38, in 
to_netcdf_via_inter_filesystem_copy
    ds.to_netcdf(source)
  File "/opt/conda/lib/python3.11/site-packages/xarray/core/dataset.py", line 
2123, in to_netcdf
    return to_netcdf(  # type: ignore[return-value]  # mypy cannot resolve the 
overloads:(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/xarray/backends/writers.py", 
line 441, in to_netcdf
    dump_to_store(
  File "/opt/conda/lib/python3.11/site-packages/xarray/backends/writers.py", 
line 491, in dump_to_store
    store.store(variables, attrs, check_encoding, writer, 
unlimited_dims=unlimited_dims)
  File "/opt/conda/lib/python3.11/site-packages/xarray/backends/common.py", line
533, in store
    self.set_variables(
  File "/opt/conda/lib/python3.11/site-packages/xarray/backends/common.py", line
571, in set_variables
    target, source = self.prepare_variable(
                     ^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/xarray/backends/netCDF4_.py", 
line 633, in prepare_variable
    datatype = _get_datatype(
               ^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/xarray/backends/netCDF4_.py", 
line 165, in _get_datatype
    return _nc4_dtype(var)
           ^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/xarray/backends/netCDF4_.py", 
line 186, in _nc4_dtype
    raise ValueError(f"unsupported dtype for netCDF4 variable: {var.dtype}")
ValueError: unsupported dtype for netCDF4 variable: StringDType(na_object=nan)

This should be fixed in the next release of xarray (pydata/xarray#11218), but for the time being we can work around it with the code in this PR.

Changes:

• Modifies the way we assign "source" coordinate values to the dataset constructed in PairedSphericalPowerSpectrumAggregator.get_dataset such that they are of a unicode dtype rather than a string dtype.

• Tests added

[Arcomano1234]

LGTM, thanks for quickly spotting this.