That should be a todo
Need function for computing sigma_33 stress

This is primarily for plane stress formulations that are not
incompressible. We need to track a separate variable for the thickness
stretching and enforce the plane stress condition.
HookesLaw::compute_33_stress_imp implemented
compute_33_stress_imp for the others.

Not implemented yet, but we don't need them right now.
Rename bool to trip adding new variable for thickness