Added attribute 'visited'
Now all children of GraphElement have an attribute visited.
attribute visited moved
It is now in the superclass GraphElement alongside with its getter and
setter.
removed TODO
constructor modified
The attribute visited is now initialized as false just like in the
AbstractVertex class.
method getEdge overriden
tests updated
added boolean attribute onThePath
It determines whether a vertex or edge belongs to a certain path or
not(in this case, to the shortest path)
method toHtml() updated
Now the vertexes that belong to the shortest path will appear as red in
the html file.
method toHtml updated
toHtml method is now the same from superclass.
edges that belong to the shortest path are also collored with red