The displacements are added to the **node_dof** records.
If **materi_displacement** is initialized, then equations like the convection
and diffusion of heat equation or the ground water flow equation are evaluated over the
deformed volume, which is the sum of the nodal coordinates plus its displacements.
Also if **materi_displacement** is initialized, the total lagrange model will
be used in stress analysis.

Condition: also **materi_velocity** should be initialized because the displacement
follows from integration of the velocity.

tochnog 2001-09-02