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.