Non-const accessor for ElasticityTensor
Stress-Strain base class

In preparation for refactoring of elasticity classes. Using a
curiously-recurring-template-pattern to define compile time
polymorphic interface for computing the stress.
Refactored interface for elasticity

Instead of having the physics compute the stress, we now template
around the stress-strain law and let that handle computing the
stress. The interface is general for curvilinear coordinate so
Cartesian versions can specialize by passing the appropriate tensors.
Also have dimension as an argument to handle manifold cases. Currently
only have Hooke's law. Hyperelasticity is coming next.