Tree Layout with vertical position constraints

[Eddykasp]

resolves #861 Implementation of a tree layout that can handle vertical position constraints for nodes. Created in the scope of the bachelor thesis of cln.

Todos:

• [ ] considerNodeModelOrder option is broken is some situations
• [ ] node sizes are not correctly determined in some cases (e.g. when using node micro layout)
• [ ] add tests

Sample ELK graph:

node wrapper {
  algorithm: org.eclipse.elk.vertiflex
  spacing.nodeNode: 5
  layerDistance: 40
  elk.padding: "[top=0.0,left=0.0,bottom=10.0,right=0.0]"
  layoutStrategy: BEND

  node n1 {
    verticalConstraint: 115
    label "n1"
  }
  node n2 {
    verticalConstraint: 400
    label "n2"
  }
  node n3 {
    verticalConstraint: 160
    label "n3"
  }

  edge n1 -> n2
  edge n1 -> n3

  node n4 {
    verticalConstraint: 300
    label "n4"
  }
  node n5 {
    verticalConstraint: 280
    label "n5"
  }
  node n6 {
    label "n6"
  }
  node n7 {
    label "n7"
  }
  node n8 {
    label "n8"
  }

  edge n3 -> n4
  edge n3 -> n5
  edge n3 -> n6
  edge n3 -> n7
  edge n3 -> n8
}

Straight edges vs preserved model order

[soerendomroes]

@Eddykasp could you add a check-list to highlight what is still missing here?

[soerendomroes]

Could you add a test for the "I do layout twice and everything breaks" problem?