Avoid leaks by hinting NewIterator methods

The iterator objects returned by vtkDataSet and derived classes
were leaking in the Python wrappers.  This patch adds hints so
that Python knows that it has to take ownership of these objects.

Common/DataModel/vtkDataSet.h

@@ -103,6 +103,7 @@ public:
   /**
    * Return an iterator that traverses the cells in this data set.
    */
+  VTK_NEWINSTANCE
   virtual vtkCellIterator* NewCellIterator();
 
   /**

Common/DataModel/vtkGenericAdaptorCell.h

@@ -202,6 +202,7 @@ public:
    * Create an empty cell iterator. The user is responsible for deleting it.
    * \post result_exists: result!=0
    */
+  VTK_NEWINSTANCE
   virtual vtkGenericCellIterator *NewCellIterator()=0;
 
   /**

Common/DataModel/vtkGenericDataSet.h

@@ -123,6 +123,7 @@ public:
    * \pre valid_dim_range: (dim>=-1) && (dim<=3)
    * \post result_exists: result!=0
    */
+  VTK_NEWINSTANCE
   virtual vtkGenericCellIterator *NewCellIterator(int dim=-1) = 0;
 
   /**
@@ -134,6 +135,7 @@ public:
    * \pre valid_dim_range: (dim>=-1) && (dim<=2)
    * \post result_exists: result!=0
    */
+  VTK_NEWINSTANCE
   virtual vtkGenericCellIterator *NewBoundaryIterator(int dim=-1,
                                                       int exteriorOnly=0) = 0;
 
@@ -143,6 +145,7 @@ public:
    * The user is responsible for deleting the iterator.
    * \post result_exists: result!=0
    */
+  VTK_NEWINSTANCE
   virtual vtkGenericPointIterator *NewPointIterator()=0;
 
   /**