确认是否该RDD已经被checkpointed过。
函数原型：
def isCheckpointed: Boolean
例子：
sc.setCheckpointDir("/home/cloudera/Documents")
c.isCheckpointed
res6: Boolean = false
c.checkpoint
c.isCheckpointed
res8: Boolean = false
c.collect
c.isCheckpointed
res9: Boolean = true