Sets the storage level to persist the contents of the DataFrame across
operations after the first time it is computed. This can only be used to assign
a new storage level if the DataFrame does not have a storage level set yet.
If no storage level is specified defaults to (MEMORY_AND_DISK_DESER)
New in version 1.3.0.
The default storage level has changed to MEMORY_AND_DISK_DESER to match Scala in 3.0.