This is a simple test of the DruckerPragerPlaneStrain elastoplastic model.
The results may be compared against the results from a 3D model with the
z-displacements constrained to zero.  There are two sets of models:  cyclic
and static. The cyclic models involve time-varying displacement BC applied
to a single cell. The static models involve initial displacements (either
axial or shear), followed by yield occurring in the time-dependent solution.

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

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

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

At present, the amount of displacement applied for the shear problem is
extremely small. Otherwise, there is additional plastic deformation after
the first time step. It remains to be seen whether this is an issue with
the code, or whether this behavior is to be expected.
