Save statistics of each partition

The original intent of calling the Statistics filter on a PartitionedDataSet was that the resulting PartitionedDataSet would have partitions matching the input giving the statics of each partition (as well as the overall statistics in global fields). There is a comment in the code that says as much.

However, the partitioned version of Execute deleted the statistics. This corrects the behavior by leaving in the partitions' statistics. This is also now being tested.