Add computation of eigenvectors of rank-2 symmetric tensor.

 Add computation of eigenvectors of rank-2 symmetric tensor.

This commit adds functions that compute the eigenvectors (and associated
eigenvalues) of a rank-2 symmetric tensor using one of threee methods:
1. Transformation to tridiagonal form and using the QL algorithm with
implicit shifting.
2. A hybrid algorithm that preferentially uses an analytical approach
and falls back to the QL algorithm if the computation is deemed
to be inaccurate.
3. The (robust but expensive) Jacobi algorithm.