always label leaf nodes

Label leaf nodes even if they don't have a row in the heatmap.
Previously, the leaf labels were only drawn after the row of the
heatmap was drawn.

Change-Id: I7d4b932fde9ce5628ed38693fd72dc8ca08ee020
hide tooltip when hovering over a blank row

Change-Id: I2cef08a3ed807923e506f16dbb1b09609444cc57
change leaf node display behavior

Prior to this commit, leaf nodes were extended so that they all
lined up vertically.  The motivation behind this behavior was
to make it easy to see which leaf nodes correponded to each
row in the heatmap.

The problem with this approach is that it obscured information
about how long these leaf nodes actually are.  Starting with this
commit, the actual length of the leaf nodes is now drawn in black
by default (the same as the rest of the tree).  The leaf nodes are
still extended as before, but this extra length is now drawn in grey.

This extension of the leaf nodes is also now a toggleable behavior.
The options are always extend leaf nodes, never extend leaf nodes,
or only extend leaf nodes if a heatmap is also loaded.  This latter
option is the default.

This commit also introduces two new public member functions: GetSize
and GetCenter.  These functions make it easier to center this item
within a scene in an end-user application.

Change-Id: I11576b8822de2c915a6edb36c80c4721661aa317
separate color maps for categorical & continuous data

This commit separates the default colors that we use to represent
categorical & continuous data within our heatmap.  Continuous data
is now drawn using a blackbody color map.  Categorical data is drawn
using the diverging accent color set.  In order to take advantage of
vtkColorSeries in this effort, a dependency on vtkCommonColor was
added to this module.

As a result of this effort, we now use one vtkLookupTable to
assign colors to all continuous data, and another for the categorical
data.  This is a simplification over our previous approach, where we
used a separate vtkLookupTable for each column.

Change-Id: If008b871c750b1020b8df04c89e34db36785f853
improve "heatmap only" performance

only draw cells and labels that are currently visible on the
screen.

Change-Id: Ic7bfa415952fc9720d199fad790b1a1115746d10
update vtkTreeHeatmapItem baselines

Update the baseline images for the tests of vtkTreeHeatmapItem.
This is necessary now that we are using separate color maps for
categorical & continuous data.

Change-Id: I5819db094fe988a579853f62f71b41aa4f66bf01
Merge topic 'label_incomplete_data' into master

633d50ab update vtkTreeHeatmapItem baselines
e60dab3a improve "heatmap only" performance
7308311a separate color maps for categorical & continuous data
1b0150a4 change leaf node display behavior
5f1dcf58 hide tooltip when hovering over a blank row
4fa063d2 always label leaf nodes