Add Parallel Graph Contraction example from lecture 3

src/main/scala/lecture3/06_ParallelGraphContraction.scala

+package lecture3
+
+import scala.collection._
+import scala.collection.parallel.CollectionConverters.MapIsParallelizable
+
+@main def parallelGraphContraction =
+  val graph = mutable.Map[Int, Int]() ++= (0 until 100000).map(i => (i, i+1))
+  graph(graph.size - 1) = 0
+  for((k, v) <- graph.par) graph(k) = graph(v)
+  val violation = graph.find {
+    case (i, v) => v != (i + 2) % graph.size
+  }
+  println(s"violation: $violation")