Adding the eggress node handling functionality

[adnan904]

• A node can be set to be a Egress node in the type attribute of the network file
• If some nodes are set as Egress then only the simulator will randomly choose one of them as the Egress node for each flow
• If some nodes are set to be Egress then once the flow is processed we check if for the flow, current node == egress node . If Yes then we depart , otherwise we forward the flow to the egress_node using the shortest_path.
• Ideally the coordination algorithms should keep the path(Ingress to Egress) of the flow in view while creating the schedule/placement. That is something we should check/implement for our DRL agent to use this feature.
• closes #135

[stefanbschneider]

Looks good if it's tested and works!

Could you add a short description/example in the readme? Or jsut link this PR, where you described how to use it.

[qarawlus]

I looked through everything. It all looks good and well written. Good job!