[FEM] add support to show absolute value result fields

- for harmonically driven forces, the results of course also have an imaginary part. Elmer outputs the real and the imaginary parts as separate result field. However, for several applications one needs the absolute (sqrt(Re^2+Im^2)
- therefore offer also absolute field if there are real/imaginary results

src/Mod/Fem/Gui/ViewProviderFemPostObject.h

@@ -103,7 +103,7 @@ public:
     // handling when object is deleted
     bool onDelete(const std::vector<std::string>&) override;
     bool canDelete(App::DocumentObject* obj) const override;
-    virtual void onSelectionChanged(const Gui::SelectionChanges &sel);
+    virtual void onSelectionChanged(const Gui::SelectionChanges& sel);
 
       /** @name Selection handling
       * This group of methods do the selection handling.
@@ -156,6 +156,7 @@ private:
                         vtkDataArray *tcoords);
     void WriteColorData(bool ResetColorBarRange);
     void WriteTransparency();
+    void addAbsoluteField(vtkDataSet* dset, std::string FieldName);
 
     App::Enumeration m_coloringEnum, m_vectorEnum;
     bool m_blockPropertyChanges;