Allowing loop on workflow

[ltdrdata]

• [x] worklist_will_execute must check visited nodes to avoid redundant
• [x] don't delete output cache unless prompt is changed in worklist_deletechanges...
  • 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.)
• [x] provide loop variable to node
• [x] determine termination of loop