node_stiffness index stiffness_x stiffness_y stiffness_z

This record adds a discrete stiffness to node index in $x$, $y$ and $z$ direction respectively. In 1D only stiffness_x needs to be specified, etc. The stiffness will lead to a nodal force of the size $stiffness\_x * u\_x$ where $u\_x$ is the displacement in $x$ direction. The same holds for the $y$ and $z$ direction. Condition: also materi_velocity_integrated or materi_displacement should be initialized.