Use arrowheads in the middle of edges in conceptual diagrams

[hanjoosten]

I asked a question at the graphviz forum the other day about arrowheads in the middle of edges, where I got a promising reply. There is a slight chance that graphviz will support arrowheads in the middle of edges. See this issue for details. When this is supported, we can generate much nicer images that shows the direction of the relation as well.

This relates to:

• https://github.com/AmpersandTarski/Ampersand/issues/610
• https://github.com/AmpersandTarski/Ampersand/issues/120