Test of Drucker-Prager plasticity using initial stresses and different
time step sizes.

A cube with initial hydrostatic pressure is subjected to simple shear
with a given loading rate. Plastic yield should occur at t = 10 years,
and should continue afterwards.

Execute the bash script runtest.sh to run both elastic and plastic models with
different time step sizes. The script then executes the plot_invars.py
Python script (requires h5py, numpy, and matplotlib). The left-hand plot is
the second plastic strain invariant as a function of time (should be zero
until t = 10 years). The right-hand plot shows the second deviatoric stress
invariant for both the elastic and plastic cases. They should be identical
until t = 10 years, at which point the plastic stresses will be less.