Generalise Top-Down Priority Queue(s) logic into a Request Manager

[SSoelvsten]

Follow-up on #397 .

In that refactor we provide a major clean-up of all the prior code. But, we can improve the code quality even more, by generalising the use of priority queues in a single class that manages all the requests.

• [ ] Use the new Request class in top-down algorithms
  • [x] count
  • [ ] pred
  • [ ] intercut
  • [x] prod2
  • [x] prod3
  • [x] quantify
  • [x] select
• [ ] Create request_manager class to centralize logic for interactions with the priority queues. This class has template arguments quite similar to request. It includes the logic
  • The type of a request.
  • The top() logic merging one or more priority queues.
  • The forwarding logic, copying the children of the current node(s) into a request placed in a per-level priority queue.
  • The level merger logic for one or more nodes.
  • The pull all ingoing arcs logic.
• [ ] Use the new Request Manager in top-down algorithms
  • [ ] count
  • [ ] pred
  • [ ] intercut
  • [ ] prod2
  • [ ] prod3
  • [ ] quantify
  • [ ] substitute