Get the slip state of grout (shear coupling spring) at end iend ∈ {1, 2}.
The return value {0, 1, 2} denotes never slipped, now slipping, or slipped in the past, respectively.
Returns:
i - slip state of grout, i ∈ {0, 1, 2} denotes never slipped, now slipping, or slipped in the past, respectively