Fix underground entries having incorrect parents. #9
Description
This is an specification of #5. The current problem that sometimes causes the error
"ValueError: Underground belt from (19, 55) to (22, 55) was preceeded by a belt at (20, 55)"
Is probably due to the parent of the entry being reassgined as a better path to that node was found. Since the "entry" itself hasn't been searched, it is not in the closed list, and so it is possible to change its parent. It hasn't been searched, since it has only been used as an entry underground, for another node.