Fixed #3514.  Added eigenvalue computation to RankTwoTensor, also
derivatives of the invariants, which will be needed for plasticity
models.

I think everything is fully tested using CPPUNIT

Thanks @jwpeterson for all your comments
Merge pull request #3515 from WilkAndy/dsyev_3514

Fixed #3514.  Added eigenvalue computation to RankTwoTensor.  Untested