adopt refactored vtkTreeHeatmapItem

Slight changes to the Visomics source code to reflect the
corresponding changes to vtkTreeHeatmapItem.  This commit
resolves a segmentation fault that occurred when navigating
to the "dendrogram only" view of a tree + heatmap in Visomics.
Merge branch 'update_tree_view'

* update_tree_view:
  adopt refactored vtkTreeHeatmapItem