Create a vector postprocessor that can sample the materials at a crack front

[lynnmunday]

Motivation

We need two be able to vary a material's fracture toughness for crack propagation in xfem.

Design

I will create a base class for the current CrackFrontNonlocalStress vector postprocessor and derive off it. I will overwrite a function that evaluates the material property at the guass point. For a scalar property, it will just return the value of the material. For a ranktwo tensor like stress, it will return the stress component normal to the crack face.

Impact

This will allow us to vary the fracture toughness in the material.