Allow saving to GraphML with mixed edge types

[nicolaspayette]

Right now, the file is produced with <graph edgedefault="directed"> or <graph edgedefault="undirected">, but the directed attribute is not set for individual edges.

If we have a mixed graph, it is not recognized as such and <graph edgedefault="undirected">.

When Jung saves to GraphML, it queries the EdgeType getEdgeType(E edge); method declared in HyperGraph. The current implementation is inherited from AbstractTypedGraph, which returns the same edge type for the whole graph.

NetLogo graphs are currently instantiated as either directed or undirected graphs. We would need to create a new class (MixedGraph?) which would inherit from our graph trait and implement getEdgeType on a 'per edge' basis.