new option to color the tree based on vertex data

New functionality introduced in this commit allows us to draw each line
in the tree with a color specified by the target vertex.  This is
accomplished by calling the new function SetTreeColorArray().

The parameter of this function specifies the name of a vtkDoubleArray
within the tree's VertexData.  The values of this array are used as the
key of a color lookup table.

Change-Id: I4c6d20f90b5791e2ce0e57ea86ad05f742a13449
Merge topic 'color_tree' into master

eac2ae01 new option to color the tree based on vertex data