Add a property to a StructArray

General Usage
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4

Ok, what missed from the example before was just the

copy_settings()

This does what you want (only for :z):

function StructArrays.component(s::StructArray, key)
    if key == :z
		return last.(getfield(StructArrays.components(s), :Z))
	end
	getfield(StructArrays.components(s), key)
end

But, if this is a good idea or good style I doubt.
E.g. what if another StructArray has some field z?