confounding edge placement

[till-tietz]

@macartan merged the changes to the plotting function into master (I added a penalty for edge intersections and a gravitation term for layout compactness to the force directed layout algorithm which makes the automated layout for more complicated DAGs when no node positions are passed a bit nicer)

last open aesthetic issue here is deciding confounding edge curvature based on graph layout. Currently all confounding edges have positive relatively symmetric curvature when negative or more skewed curvature may be optimal depending on the layout.

I don't think this is super critical to implement but I'll keep tinkering.