A type for receiving the results of deserializing TDigestUDT.
The payload is the tdigest member field, holding a TDigest object.
This is necessary because (a) I define the TDigest type is defined in the isarn-sketches
package and I do not
want any Spark dependencies on that package, and (b) Spark has privatized UserDefinedType under
org.apache.spark scope, and so I have to have a paired result type in the same scope.
The TDigest payload, which does the actual sketching.