simplify probes, use pyVTK library
Primer - use VTK probes
new postrocessing function base on VTK, example
update User's guide - postrocessing filters
Merge pull request #276 from vlukes/post_vtk
fix get_vtk_from_mesh(), remove temporary file
update test_install.py for current linear_elastic_probes.py
prevent modifying linear_elastic.py namespace in derived examples
new tests/test_input_linear_elastic_probes.py