Guiding Backtrack in DFSExplorer
DFSExplorer now saves an priority_queue based on the guide distance of the state. This queue contains every opened point of the exploration tree so that when the algo backtracks, it can pick the best candidate to do it, but still, eventually, try everything.
There is also the implementation of a simple guide different from the basic one, so we can test that changing the guiding method in the configuration isn't an issue.