Third invariant added to RankTwoTensor

Derivatives of it, and of second invariant added.

Tested using CPPUNIT

Fixes #3645
Merge pull request #3646 from WilkAndy/third_invariant_3645

Third invariant added to RankTwoTensor
Merge commit '3b8849cb84e5d16d625d4ced46f72a9cacbd95a6'