[!836] Fix computation of eigenvectors of singular matrices

Merge branch 'eigenvalues-of-singular-matrices' into 'master'

ref:core/dune-common\> The eigenvector algorithm for symmetric FieldMatrices
was buggy, it does not handle singular matrices well. Add a test and fix the
bug.

See merge request [!836]

  [!836]: gitlab.dune-project.org/core/dune-common/merge_requests/836