Use the new function vtkAbstractArray::SetMaxDiscreteValues within
vtkTreeHeatmapItem.  This allows us to properly color categorical
datasets that contain more than 32 distinct values.

Within this commit, we also change the color set that we're using for
the categorical color map.  The accent set is pretty, but it only has
8 different values.  Instead we'll use qualitative set3 because it
has 12 values.

Finally, we fix a bug in vtkTreeHeatmapItem so that our NaN color
(grey) is used appropriately by our categorical lookup table.

Change-Id: I53bc9f13240be8c0c10b5dbc77bdcca1ea0fa2a1