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.
