[FEM] importVTKResults: assure user gets a visible object

- on importing a .vtu/.vtk file the new created should be a visible one. Therefore set its display to surface like we do when a new pipeline is created from within FC by the user

src/Mod/Fem/feminout/importVTKResults.py

@@ -134,6 +134,9 @@ def importVtkVtkResult(
 ):
     vtk_result_obj = FreeCAD.ActiveDocument.addObject("Fem::FemPostPipeline", resultname)
     vtk_result_obj.read(filename)
+    # set display mode to "Surface" like for any other new pipeline to assure the user sees
+    # something after the import was done
+    vtk_result_obj.ViewObject.DisplayMode = "Surface"
     vtk_result_obj.touch()
     FreeCAD.ActiveDocument.recompute()
     return vtk_result_obj