This is a simple test of the PowerLawPlaneStrain viscoelastic model. The
results may be compared against the results from a 3D model with the
z-displacements constrained to zero.  There are six models:

Plane strain axial displacement problem:
pylith planestrain2d.cfg axialdisp2d.cfg

Plane strain shear displacement problem:
pylith planestrain2d.cfg sheardisp2d.cfg

Plane strain axial displacement problem with initial stresses:
pylith planestrain2d.cfg axialdisp_init_stress_2d.cfg

3D simulated plane strain axial displacement problem:
pylith planestrain3d.cfg axialdisp3d.cfg

3D simulated plane strain shear displacement problem:
pylith planestrain3d.cfg sheardisp3d.cfg

3D simulated plane strain axial displacement problem with initial stresses:
pylith planestrain3d.cfg axialdisp_init_stress_3d.cfg

The 2D and 3D problems should yield the same displacements, strains,
stresses, and viscous strains when the appropriate components are compared.
