Get the yield state of the shear coupling spring at node inode (inode ∈ {1, 2}).
Return a value i ∈ {1, 2, 3} denotes never yielded, yielding now, or yielded in the past, respectively.
Returns:
i - yield state, i ∈ {1, 2, 3} denotes never yielded, yielding now or yielded in past, respectively