Parse edge control points when converting from dagitty syntax

[jtextor]

dagitty allows to specify a single control point when drawing edges, e.g.

dag {
bb="-3,-0.5,2,1.2"
D [outcome,pos="1.000,1.000"]
E [exposure,pos="-2.000,1.000"]
Z [pos="-0.500,0.500"]
D <-> Z [pos="1.000,-1.000"]
E -> D
E <-> Z [pos="-2.000,-1.000"]
}

The edge control points are currently not imported by ggdag, which would be convenient for users who use the dagitty web interface to initially build dogs.

[malcolmbarrett]

Note to self on this: rather than trying to replicate exactly what the user sees in the web interface, which would be a difficult game to get into, maybe the control point information can be transformed into a curvature value for curved edges