Allowing loop on workflow

[Dr.Lt.Data]

• worklist_will_execute must check visited nodes to avoid redundant
• don't delete output cache unless prompt is changed in worklist_delete_changes_...
  • If a node in the middle of a loop is edited, all results after that point are invalidated. (This includes the starting point of the loop, as it becomes a middle node of the loop.)
• provide loop variable to node
• determine termination of loop