Traverser traverses all edges

[flo-dup]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem ? No

What kind of change does this PR introduce? Bug fix

What is the current behavior? According to javadoc UndirectedGraphImpl::traverse(int v, Traverser t) should traverse the entire graph. It traverses all the vertices but not all the edges.

What is the new behavior (if this is a feature change)? UndirectedGraphImpl::traverse(int v, Traverser t) traverses all the vertices and all the edges of the graph.

Does this PR introduce a breaking change or deprecate an API? No

[flo-dup]

Needs to be discussed!

Indeed, if we want all the edges to be traversed but to be traversed each only once, we need a boolean array for keeping track of edges encountered.