support Newick files with no edge weights

Update the vtkNewickTreeReader to more gracefully handle files where
no edge weights were specified.  As of this commit, each edge will
be given a weight of 1.0 in this case.  Previously, all edges were
given a weight of 0.0, and this caused problems with
vtkTreeLayoutStrategy.

Change-Id: I77eae99b1d39409ab4d1f242c17d51fe523c6c3d
Merge topic 'no_weights_tree' into master

e9a5d4fb support Newick files with no edge weights