improve performance when reading a large Newick tree

Use vtkTreeDFSIterator when calculating the node weights of a tree
ready by vtkNewickTreeReader.  For a sample tree with 63498 vertices,
this change improves read time from 70.47s to 4.14s

Change-Id: I0e35ff89ce207657a958cf606a85984aa8ee3de0
Merge topic 'improve_big_tree_reader' into master

907df317 improve performance when reading a large Newick tree