Improve case_input.nc output handling

[Sandro-Meier-Empa]

This MR improves save_case_input() by allowing overwrites and preserving metadata from xarray inputs.

Changes

• Preserve xarray.DataArray.attrs when writing variables to NetCDF.
• Add an overwrite=False argument to save_case_input().
• Raise FileExistsError when the target input file already exists unless overwrite=True.
• Use pathlib.Path for output file path handling.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR enhances save_case_input() to better support xarray metadata and safer file writes by adding an overwrite guard and propagating .attrs into the generated NetCDF output.

Changes:

• Replace os.path path handling with pathlib.Path, and add an overwrite flag to control clobbering existing output files.
• Introduce add_attrs() to persist xarray-style attributes onto NetCDF variables/groups.
• Update variable writing to extract .values for xarray.DataArray inputs and attach attributes.

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