ScoreSettable is an interface that let caller set external scores for
the heuristic. The ExternalScoreAttachment and WeightedCombAttachment
heuristics implement this interface.
This commit adds a method SetNodesScores to the WeightedCombAttachment
heuristic.
Since the heuristic keeps a list of sub-heuristics, it will attempt to
recursively apply the scores to the sub heuristics.
This commit defines a new heuristic WeightedCombAttachment that takes a
set of sub-heuristics, and produces a final node score by querying the
sub-heuristics and combining the scores from them according to their
weights.
This way it will look like a regular, single heuristic to the autopilot
agemnt, but can be a more complex combination of several.
