Fix Python PCA test harness interface

[SauersML]

Summary

• teach the Python reference script to accept both --n-components and --n_components
• replicate the crate's centering and sample-standard-deviation scaling inside the Python helper
• reuse the shared preprocessing in both the manual and library PCA code paths used by the tests

Testing

• cargo test pca_tests::test_pca_2x2 --test pca_tests

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https://chatgpt.com/codex/tasks/task_e_68d362ccd098832e9df3e619b7f067db