Fix index parsing in mpas_to_xdmf tool

[xylar]

This pull request focuses on improving the robustness of the _parse_indices function and significantly expanding the test coverage for the MPAS-to-XDMF conversion functionality. Key changes include handling edge cases in index parsing, adding validation, and introducing comprehensive unit tests for various scenarios.

Enhancements to _parse_indices function:

• Improved handling of slice notation by validating the number of colons and padding missing parts to ensure consistent parsing. Added explicit error handling for invalid index strings.

Expanded test coverage:

• Added unit tests for _parse_indices to validate both valid and invalid cases, including mixed slice/list formats, empty strings, and invalid strings.
• Introduced tests for loading MPAS mesh datasets, handling time series, and processing extra dimensions, ensuring compatibility with edge cases like missing variables or invalid configurations.
• Implemented tests for the CLI entry point (main) with simulated arguments and user input, verifying the creation of expected output files.

[Copilot]

Pull Request Overview

This PR enhances the _parse_indices function to explicitly handle slice notation by splitting the index string into up to three parts, padding missing values, and applying defaults for start, stop, and step.

• Replace one-liner list comprehension with explicit parsing for slice support
• Preserve comma-separated index behavior unchanged

Comments suppressed due to low confidence (2)

conda_package/mpas_tools/viz/mpas_to_xdmf/io.py:300

• Update the function docstring to describe the new slice notation support (start:stop:step semantics) and default behaviors when parts are omitted.

def _parse_indices(index_string, dim_size):

conda_package/mpas_tools/viz/mpas_to_xdmf/io.py:312

• Add unit tests for _parse_indices covering edge cases like ':', '5:', ':10', '::2', and invalid formats to ensure the new parsing logic works as expected.

return list(range(start, stop, step))

[xylar]

Testing

I was able to extract kiteAreasOnVertex with vertexDegree=:, vertexDegree=0:3, and vertexDegree=0:3:1 with this fix, whereas it crashed without the fix.