Solid mechanics variables class
Solid mechanics FE variables
Build solid mechanics variables
Update init interface for SolidMechancs variables.

We need this for cases where the elements are manifolds in space.
libMesh::Elem::mesh_dimension() only reports the manifold dimension
and libMesh::MeshBase::spatial_dimension is a compile time constant.
Thus, we need to provide an interface to initialize the displacement
variables that we'll need. Still have to think about to actually
determine the dimension at runtime...
Mostly complete first draft of ElasticMembrane.

Haven't included it in the build system so therefore there's still
work to do.
Comment
Should be a minus sign
Template on ElasticityTensor type.

Also need the strain tensor to compute the stress.
First draft for computing stress.

Now to implement some ElasticityTensors.
First draft of Hooke's law.
Compile time fixes for HookeanElasticity
Introduce base class for elasticity tensors
Have Hooke's law use base class
Make function inline
Updated the API for the elasticity tensor
Instantiate ElasticMembrane.

Whenever we have new nonlinear elasticity stuff, we can instantiate
in this file. Idea is that if user extends ElasticityTensor, they won't
have to rebuild these things, only their part.
Missing functions in SolidMechanicsVariables
We want the FE variables version
Missing headers
Consistent private variable naming