[FEM] make pvtu file filtering optional

- it is sensible to filter by default, however, it turned out that on some complex models Elmer fails to compute if the mesh volume regions are too small (in most cases) or at a certain mesh region. By disabling the filtering, one gets at least for the latter case a visual feedback where the mesh volume of the different CPU are (by also setting a transparency to the result pipeline).

src/Mod/Fem/Gui/ViewProviderFemPostObject.cpp

@@ -706,9 +706,14 @@ void ViewProviderFemPostObject::filterArtifacts(vtkDataObject* data)
     m_wireframe->SetInputData(data);
     m_points->SetInputData(data);
 
-    // filter artifacts
-    // only necessary for the surface filter
-    filterArtifacts(data);
+    // filtering artifacts is only necessary for the surface filter
+    auto hGrp = App::GetApplication().GetParameterGroupByPath(
+        "User parameter:BaseApp/Preferences/Mod/Fem/Elmer");
+    bool FilterMultiCPUResults = hGrp->GetBool("FilterMultiCPUResults", 1);
+    if (FilterMultiCPUResults)
+        filterArtifacts(data);
+    else
+        m_surface->SetInputData(data);
 
     return true;
 }