change signatures for wrapping

Apparently functions whose signatures contain vtkSmartPointer
are not wrapped into python.  Change such methods of
vtkTreeHeatmapItem to contain ordinary pointers instead.

Change-Id: Ic98311c10f1e7a99fb4ad071d3981e4febb0604e
Merge topic 'treeheatmapitem_wrapping' into master

6d501034 change signatures for wrapping