[FEM] enable to run Elmer multi-threaded

- this needs proper testing, especially on a non-Windows system

- note that for some tasks multi-threading requires non-standard additional solvers like MUMPS. Ideally the user should be informed about this, depending on the equations he uses. But this should not block this PR, meaning to use multi-threading in general.

src/Mod/Fem/App/FemVTKTools.cpp

@@ -53,6 +53,7 @@
 # include <vtkRectilinearGrid.h>
 # include <vtkUnstructuredGrid.h>
 # include <vtkXMLUnstructuredGridReader.h>
+# include <vtkXMLPUnstructuredGridReader.h>
 # include <vtkXMLUnstructuredGridWriter.h>
 # include <vtkPointData.h>
 # include <vtkCellData.h>
@@ -215,6 +216,14 @@ FemMesh* FemVTKTools::readVTKMesh(const char* filename, FemMesh* mesh)
         }
         importVTKMesh(dataset, mesh);
     }
+    else if (f.hasExtension("pvtu")) {
+        vtkSmartPointer<vtkDataSet> dataset = readVTKFile<vtkXMLPUnstructuredGridReader>(filename);
+        if (!dataset.Get()) {
+            Base::Console().Error("Failed to load file %s\n", filename);
+            return nullptr;
+        }
+        importVTKMesh(dataset, mesh);
+    }
     else if(f.hasExtension("vtk"))
     {
         vtkSmartPointer<vtkDataSet> dataset = readVTKFile<vtkDataSetReader>(filename);