Add NDCube._extra_attrs_to_copy for subclass attribute propagation

[nabobalis]

PR Description

Subclasses can declare instance attributes that are automatically propagated (by reference) to cubes derived through _new_instance (arithmetic operations) and through to_nddata when the target type is a subclass carrying the same attributes. This removes the need for subclasses to override both methods just to keep domain-specific metadata alive on derived cubes. Slicing is deliberately not covered, as shape-dependent attributes cannot be sliced generically.

This is a controversial change and due to something I have to do in irispy as I have attached (to Stuart's horror) lots of new attrs to make my life easier.

I think this will not be merged in, and the fix is for me to attach more of the attrs into the metadata instead.

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