#175 DelegateTree does not calculate depths of backing graph

[Gert van Valkenhoef]

According to the javadoc of
public DelegateTree(DirectedGraph<V,E> graph) {
the given backing graph must be a tree (i.e. is allowed to be non-empty). However, as the attached unit test shows, this breaks the depth information in DelegateTree, as this information is not calculated for any pre-existing edges.

[Gert van Valkenhoef]

Test breaking getDepth in DelegateTree