incorrect `iterate` override

[nsajko]

It's not correct to "only iterate over stored values":

NDimensionalSparseArrays.jl/src/ndsparsearray.jl

Lines 132 to 145 in 6fde3dc

	# Iteration - only iterates over stored values
	function Base.iterate(A::NDSparseArray)
	iter_state = iterate(A.data)
	isnothing(iter_state) && return nothing
	(idx, val), state = iter_state
	return (val, state)
	end
	function Base.iterate(A::NDSparseArray, state)
	iter_state = iterate(A.data, state)
	isnothing(iter_state) && return nothing
	(idx, val), new_state = iter_state
	return (val, new_state)
	end