[DYNOSuprovo]

Fixes #457

Summary of Changes

• Implement __array_finalize__(self, obj) in SpectralGradient to preserve wave0 and wave attributes when slicing, indexing, or copying instances.
• In SpectralGradient.__new__, preserve existing wave and wave0 attributes when converting or re-instantiating SpectralGradient instances unless explicitly overridden.
• Add test coverage in sbpy/spectroscopy/tests/test_specgrad.py verifying that scalar indexing, subarray slicing, copying, and re-instantiation preserve wave0 and wave.
• Update CHANGES.rst under v0.7.0 (unreleased) -> Bug Fixes -> sbpy.spectroscopy.

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