Fleshed out exodiff functionality for MultiPhysicsCrystalPlasticity/SingleElement2d tests.

Expanded run_exodiff_test.py script, modified CMakeLists.txt files to copy gold.exo and
.exodiff files to the bin test directory, etc.  The tests now pass or fail based on a
combination of the return code from AlbanyT and the return code from exodiff.  Exodiff
checks displacement, force, Cauchy_Stress, Fp, and Velocity_Gradient.

Also, fixed a glitch in MultiPhysicsCrystalPlasticityModel_Def.hpp that resulted in an
uninitialized Velocity_Gradient for the elastic case (led to NaNs in the .exo file).