Add traverse_graph / AGE engine for visualization

The graph visualization feature now uses the traverse_graph function
(with AGE as the main engine) to collect the requested nodes to be
visualized. This was implemented in the methods of the graph class:
previously, `recurse_descendants` and `recurse_ancestors` used to
work by calling `add_incoming` and `add_outgoing` many times, which
in turn have to load nodes during the procedure. Now these are all
independent and they all call the traverse_graph function, so the
information is obtained directly from the query projections and no
nodes are loaded. So these changes are not only important as a first
step to homogenize graph traversal throughout the whole code: an
improvement in the visualization procedure is expected as well.