Flags for controlling the storage of an RDD. Each StorageLevel records
  whether to use memory, whether to drop the RDD to disk if it falls out of
  memory, whether to keep the data in memory in a serialized format, and 
  whether to replicate the RDD partitions on multiple nodes. Also contains 
  static constants for some commonly used storage levels, such as 
  MEMORY_ONLY.
    |  | DISK_ONLY = StorageLevel(True, False, False, False) | 
    |  | DISK_ONLY_2 = StorageLevel(True, False, False, False, 2) | 
    |  | MEMORY_ONLY = StorageLevel(False, True, False, True) | 
    |  | MEMORY_ONLY_2 = StorageLevel(False, True, False, True, 2) | 
    |  | MEMORY_ONLY_SER = StorageLevel(False, True, False, False) | 
    |  | MEMORY_ONLY_SER_2 = StorageLevel(False, True, False, False, 2) | 
    |  | MEMORY_AND_DISK = StorageLevel(True, True, False, True) | 
    |  | MEMORY_AND_DISK_2 = StorageLevel(True, True, False, True, 2) | 
    |  | MEMORY_AND_DISK_SER = StorageLevel(True, True, False, False) | 
    |  | MEMORY_AND_DISK_SER_2 = StorageLevel(True, True, False, False, 2) | 
    |  | OFF_HEAP = StorageLevel(False, False, True, False, 1) |