Added extra slice hook useful for downstream

[nabobalis]

PR Description

any NDCube subclass holding axis-tracking state that NDMeta's generic slicing can't express (per-frame WCS lists, instrument lookup tables that change meaning when an axis collapses) currently must override getitem, duplicate the mixin's ellipsis/sanitize logic, and hope upstream slicing internals don't shift under them. One no-op hook removes that entire class of override.

ALL TO MAKE MY IRISPY PR SMALLER.

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AI tools were used for:

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• Test/benchmark generation
• Documentation (including examples)
• Research and understanding
• No AI tools were used

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