Support undirected views

[miratepuffin]

There are several instances where it is a pain to be dealing with a directed graph - i.e. we just want to know if an edge exists between two nodes, but we have to check both directions. As such we should support a view on the graph where edges direction is ignored.

There is some semantic questions here on if we 'combine' edges that exist in both directions, what happens to their properties.