Get the yield state of the coupling normal spring at end iend ∈ {1, 2} of the pile element.
Return value ∈ {0, 1, 2} denotes never yielded, now yielding, or yielded in the past, respectively.
Returns:
i - value ∈ {0, 1, 2} denotes never yielded, now yielding, or yielded in the past, respectively