Shortcut finding optimisation

[Caruychen]

Further optimisation to the dynamic programming in bfs search.

The bfs search is allowed to revisit some visited, and free nodes (free being not on an existing path), if it considers it a shortcut.

Bfs determines the next node will be a shortcut IF:

• The next node was previously visited through a trail extending from the same path, as the current node.
• the next node's dst_to_start is the current node's dst_to_Start + 1
• the next node was visited through a trail closer to the end node than the current node's trail.

[crl-n]

After running the bugfinder a few times I found a map that segfaults. We can see if that is an easy fix or we can just go with what we have in main so far, it will be enough to pass the project.