fix: load pre-7.0 clustered NetCDF files with legacy original_data refs

[FBumann]

Problem

Loading a clustered FlowSystem from a NetCDF file written by a pre-7.0 flixopt fails with:

OSError: Failed to load FlowSystem from NetCDF file ...:
Referenced DataArray 'original_data|...|fixed_relative_profile' not found in dataset

Files written before flixopt 7.0 stored clustering.original_data / clustering._metrics as
:::original_data|... / :::metrics|... references in the clustering attrs JSON. Those target
arrays are no longer serialized (they only fed the removed plot.compare()), and the current
Clustering.__init__ also rejects the _original_data_refs / _metrics_refs keys outright. So
_restore_clustering — which resolves the structure against an empty arrays dict — raises.

Fix

Drop the two legacy keys in _restore_clustering before resolving. Files written by 7.x are
unaffected (they emit no such references).

Tests

Adds TestLegacyClusteringBackwardCompat (in-memory + real NetCDF roundtrip). Verified the new
tests fail without the fix (reproducing the exact error) and pass with it.

Release note

Already shipped to users on the 7.2.x line as 7.2.2 (tag v7.2.2, off release/7.2). This PR
forward-ports the same fix to main so it isn't lost in the next (8.0.0) release.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between cc56774 and 6201fcc.

📒 Files selected for processing (2)

• flixopt/io.py
• tests/test_clustering/test_clustering_io.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/legacy-clustering-load-main

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