Update to starcost when choosing center cluster of overall spanning tree
to use Dr. Bui's mentioned cost/(height of subtree at the new cluster). 

We are now using cost^3 for selection of which edge to add a new cluster
to the overall spanning tree.

Note: The parent/child relation was fixed for the newly added clusters
to the spanning tree after a root was chosen for their subtrees. This
resulted in the last overall spanning tree graph looking as it should.