autopilot/interface+externalscoreattach: define ScoreSettable

ScoreSettable is an interface that let caller set external scores for
the heuristic. The ExternalScoreAttachment and WeightedCombAttachment
heuristics implement this interface.

autopilot/combinedattach.go

@@ -25,7 +25,7 @@ type WeightedCombAttachment struct {
 
 // NewWeightedCombAttachment creates a new instance of a WeightedCombAttachment.
 func NewWeightedCombAttachment(h ...*WeightedHeuristic) (
-	AttachmentHeuristic, error) {
+	*WeightedCombAttachment, error) {
 
 	// The sum of weights given to the sub-heuristics must sum to exactly
 	// 1.0.
@@ -44,8 +44,9 @@ func NewWeightedCombAttachment(h ...*WeightedHeuristic) (
 }
 
 // A compile time assertion to ensure WeightedCombAttachment meets the
-// AttachmentHeuristic interface.
+// AttachmentHeuristic and ScoreSettable interfaces.
 var _ AttachmentHeuristic = (*WeightedCombAttachment)(nil)
+var _ ScoreSettable = (*WeightedCombAttachment)(nil)
 
 // Name returns the name of this heuristic.
 //
@@ -135,6 +136,8 @@ func (c *WeightedCombAttachment) NodeScores(g ChannelGraph, chans []Channel,
 //
 // Since this heuristic doesn't keep any internal scores, it will recursively
 // apply the scores to its sub-heuristics.
+//
+// NOTE: This is a part of the ScoreSettable interface.
 func (c *WeightedCombAttachment) SetNodeScores(targetHeuristic string,
 	newScores map[NodeID]float64) (bool, error) {