FEM: unit tests, add tests for von mises and prinzipal stress calculation
FEM: unit tests, add tests for von mises and prinzipal stress calculation

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master