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.
Add method to viscosity for evaluating at a point.

This is introducing a bunch of warnings about ambiguity of types, see
http://stackoverflow.com/questions/3519282/why-is-this-ambiguity-here
Need to get rid of the old interface that's being used in
LowMachNavierStokes. Was hoping to avoid recomputing temperature -
need to think about the right API for that.
Add postprocessing computation of viscosity

Still need to register the variable before it will be output.
Register viscosity for postprocessing.

Viscosity should be plottable now.
Need to actually set the bloody value.
Don't have default value for time.

This was causing warnings in opt mode, but causing *errors* in dbg
mode. We can pass time anyway from the context, so it's not a big
deal. We can update the API later.
Need to *not* do this initialization
Add API to conductivities for postprocessing/visualization
Compute k for postprocessing

It is not enabled yet.
Register k for postprocessing

This should enabled visualization of conductivity postprocessing.
Merge pull request #176 from pbauman/mu-k-plotting

Viscosity and conductivity visualization
Cannot "initialize" value in the postprocessing computation

See discussion in #175.
Merge pull request #177 from pbauman/vis-value-fixes

Cannot "initialize" value in the postprocessing computation
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
Update compute_metric_tensors API

Need context for upcoming variable for tracking thickness extension
in compressible materials.
Add variable for compressible materials

When the material is compressible, then the thickness extension
is not implicit and must be tracked as a separate DoF. This
add that capability.
Principal stress postprocessing.