The PortConstraintEnforcementStage enforces that a layout algorithm will not change
the relative coordinates of ports that are associated with a strong/fixed PortConstraint.
Usage:
//prepare port constraints
graph.addDataProvider(PortConstraintKeys.SOURCE_PORT_CONSTRAINT_KEY, createSPCs(graph));
graph.addDataProvider(PortConstraintKeys.TARGET_PORT_CONSTRAINT_KEY, createTPCs(graph));
//create layout pipeline
OrganicLayouter layouter = new OrganicLayouter();
PortConstraintEnforcementStage stage = new PortConstraintEnforcementStage();
stage.setCoreLayouter(organic);
//launch algorithms
new BufferedLayouter(stage).doLayout(graph);