Helper function for building an FE

Note this is copied from libMesh::FEMContext because it's private
there. Should consider friending it or making that function public.
First draft of stress and strain postprocessing for ElasticMembrane

Need to collect some of this into a helper function because there's
some code duplication going on.
Enable postprocessing for ElasticMembrane.
ElasticMembrane always has \lambda coupled

This is because we're always assuming plane stress with this. However,
if the material is incompressible, we can explicitly compute \lambda.
If not, then we can and it's tracked as an independent variable that
we need to discretize.
Helper function for computing metric tensors
Use metric tensor helper function
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
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.
Merge pull request #180 from pbauman/elastic-membrane-updates

Elastic membrane updates