Build DAG then create an ordered list of gates from it. DAG starting
points could be Net or Instances (we take the output of the Instance).
Multiple DAG can be created, but once an Instance is part of a DAG,
it connot be part of another and is conidered as "reached" by all
subsequent DAG run. The ordered list build from a DAG contains both
Net and Instances.
A new property has DagProperty, accessible through DagExtension
has been created to store relevant information of the DAG on each
Instance or Net (work in progress).
Robert Taylor
Jean-Paul Chaput