nonlinear spring law of examples/compare/spline_spring.fds
   x [m]     id   F [N]
-0.2  1   600.0
-0.1 2 150.0
  -0.05   3   50.0

0.0 4 0.0
0.05 5 -40.0
0.1   6  -120.0
0.2 7 -500.0
