display scalar bar for colored tree

vtkDendrogramItem now displays a vtkColorLegend after SetColorArray()
has been called.  This allows the consumers of this visualization to
see the scalar values associated with the colored edges of the tree.

This commit also contains a bug fix.  When SetColorArray() is called
with an array that only contains a single value, the edges of the tree
should be drawn in grey.  This bug previously caused such edges to be
drawn in either red or blue.

Change-Id: Ia897109b119029e57e053b368dc910fb87266a76
map tree #1's root to tree #2's root

This fixes a off-by-one bug.  This mapping has to be set explicitly
because the subsequent recursive mapping generator needs to stop
just short of the root.

Change-Id: If749f88a6a7fb7b036ea49e81637187eaf14ad0d
Merge topic 'improve_dendrogram_color_array' into master

0b0777a0 map tree #1's root to tree #2's root
f50733ec display scalar bar for colored tree