always write tree edges in incremental order

Prior to this commit, vtkTreeWriter assumed that the edges of its
input tree would appear in child order.  Instead, we now explicitly
write the edges in incremental order, querying the tree for the
source & target vertices for each edge as we go.

Change-Id: I776a5386ef41bd014cdd2a6cee30d29646741f8e
Merge topic 'fix_tree_edge_order' into master

4cadcb67 always write tree edges in incremental order