mark as scalaGraph
simplify addVertex and addEdge
override toString
use ScalaVertex for Graph.V and Pipeline.V -> ScalaElement needs to
extend Element; next problem: ScalaVertex needs to extend Vertex!
ScalaVertex must extend Vertex, ScalaEdge must extend Edge in order to
reuse existing pipes
graph.E now iterates over ScalaEdge
only use ScalaVertex and ScalaEdge in Graph
clean up, use ScalaEdge where possible