Average stress (hydrostatic compressibility)

An extra average stress contribution on each of $\sigma_{11}$, $\sigma_{22}$ and $\sigma_{33}$ is

$$\frac{1}{co} \frac{\partial v_i}{\partial x_i}$$

where $co$ is the group_materi_elasti_compressibility, which should not be 0. This pressure term can e.g. be used to model nearly incompressible fluids. The compressibility contribution should be combined with a contribution for the deviatoric stresses (e.g. group_materi_viscosity).