Change to xarray for indexing

[aidanheerdegen]

Change to xarray for indexing to allow support for more file formats.

xarray.open_dataset for indexing would supports more file formats, particularly zarr, grib and UKMO pp files (via Iris). Would allow for transparent data access regardless of format. Would also make it simple to also support accessing data via cloud stores, e.g. Amazon S3 with zarr.

See https://docs.xarray.dev/en/stable/user-guide/io.html for more information on back-end format support in xarray

This would almost certainly end up requiring a change to the schema to support different file formats that may not have concepts such as chunk size.