wrap more of vtkTreeHeatmapItem's public API

This commit introduces minor API changes to vtkTreeHeatmapItem.
Instead of using pointer arguments, we now pass in arrays.
This allows GetCenter() and GetSize() to be used from Python
wrapped VTK.

Change-Id: If223e57f814e0e01a8604caad70ffd5d4d614079
Merge topic 'wrap_more_functions' into master

7f0544bd wrap more of vtkTreeHeatmapItem's public API