fixed computing of gain

src/main/scala/org/scadcop/solver/decentralized/mgm2/MGM2Mind.scala

@@ -105,7 +105,7 @@ class MGM2Mind(val context : Context = new Context(),
             myValue != context.getValue(potentialPartner).get) { // If this is a move from the current context
             val potentialContext: Context = context.fix(variable, myValue).fix(potentialPartner, partnerValue)
             val potentialCost: Double = potentialContext.cost(constraints)
-            val gain = currentCost - potentialCost
+            val gain = potentialCost - currentCost
             val move = new BilateralMove(potentialContext, gain)
             moves ::= move
           }