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 '14294-calculator-normals-tcoords' into pv-master

8c12163 Enable vtkArrayCalculator to generate normals and texture coordinates.
Merge topic 'pvweb-test-images-to-dashboard' into pv-master

850dc80 Support providing a temp dir so that pvweb test images go to dashboard.
Merge remote-tracking branch 'pvvtk/pv-master' into HEAD

Change-Id: I0bc320abad9229b1676fc4d767bf1194e8a43181
Merge topic 'fix_tree_edge_order' into master

4cadcb67 always write tree edges in incremental order