[GraphView]: Add getOrderedNodes() method for topological order

Add getOrderedNodes(reversed=false) method which produces
a topological order of the graph view (or of the reversed
graph view when reversed is true).
This order is also deterministic given the nodes set and
the ordered output nodes.
Cyclic graphs are handled by breaking back edges as declared
in the new setBackEdges() method of the Operator class.
The order is suitable for resolving constant propagation
of tensor types in one pass including for cyclic graphs
if back edges are declared correctly for operators.
It is also suitable for exporting to onnx graphs
which require a topological order of exported acyclic graphs.