Add visualization block for stress eigenvectors

The new visualization block writes a stress eigenvector of choice into the VTK file. The schema for the visualization is:

visualization:
  _blockname: visualization_0
  blocks:
    vis_stress:
      _blockname: vis_stress_0
      vector: 0
      index: 0  

The eigenvectors are stored according to the corresponding eigenvalue in ascending order. The default index is dim - 1, which is the eigenvector of the largest eigenvalue. The block is added to the registry. The entry in the VTK file is called stress_ev_<idx>, where idx is the index selected.

Changes:

• Add StressEVVisualizationBlock using a new StressEVGridFunction.
• Register the visualization block.
• Add visualization of stress eigenvectors to experiments/hansbo_reproduction/fig23_3.yml, which is used in the tests.