Add vertical springs to check upheaval forces

For some pipeline models, it might be more flexible to use vertical springs to suppress upheaval than restraining displacements in a certain direction.

One typical example is shown below [N, mm]:

*spring, elset=Vertical-Springs, nonlinear

3,

0.0, -19.0

0.0, 0.0

75.0, 20.0

7500.0, 60.0

101020.0, 200.0

**

*ELEMENT, TYPE = SPRING1

990000001, 1

*ELGEN, ELSET = Vertical-Springs

990000001, NumOfNodes, 1, 1

Remember that only remove/add the vertical spring at the proper step.