Group index helpers, add index accessors, and move copying separator

[rguliev]

Motivation

• Fix the misplaced Copying section separator so index-related helpers are grouped together for clearer code organization.
• Provide explicit index accessors and ensure set_index/reset_index are contiguous and behave consistently with SpectraFrame copy semantics.
• Remove the redundant __getattr__ special-case for index now that an index property is present.

Description

• Move the # Copying separator to appear after the index helper methods and correct the section header to # Index.
• Add an index property and setter, and keep set_index and reset_index implementations that return new SpectraFrame objects and ignore inplace.
• Remove the __getattr__ clause that returned self.data.index for the index attribute since the property now covers that behavior.
• Add tests in tests/test_spectra.py to cover index setter behavior and set_index/reset_index methods and adjust test_getattr expectations.

Testing

• No automated tests were executed as part of this change.
• New and modified tests live in tests/test_spectra.py but were not run in this rollout.

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Codex Task

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