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
Not computing Jacobians yet
Compile time fixes