FEM: Multiframe adoptions

- To support timedata and the relevant filters the pipeline needs to be fully setup, hence not only working on data
 - Multiblock source algorithm is needed to supply the time data for the algorithms

src/Mod/Fem/App/FemPostObject.h

@@ -27,6 +27,7 @@
 #include "PropertyPostDataObject.h"
 #include <App/GeoFeature.h>
 #include <vtkBoundingBox.h>
+#include <vtkDataSet.h>
 
 
 namespace Fem
@@ -45,6 +46,12 @@ public:
 
     Fem::PropertyPostDataObject Data;
 
+    // returns the DataSet from the data property. Better use this
+    // instead of casting Data.getValue(), as data does not need to be a dataset,
+    // but could for example also be a composite data structure.
+    // Could return NULL if no dataset is available
+    virtual vtkDataSet* getDataSet();
+
     PyObject* getPyObject() override;
 
     vtkBoundingBox getBoundingBox();