Bug: some model solutions do not get 10/10 route scores

[evemartin]

The current route scoring algorithm provides undesired route scores for some Python levels, in particular Python Den Level 38. For this level, because of the level's learning goal, the model solution involves taking a slightly longer route as the van has to turn around and double back on itself - because of this the route score for the model solution is 8/10.

This task is to investigate the current route algorithm and ensure that all model solutions get a 10/10 route score.