New operator map partition function

I think this comment is not quite correct. Something more appropriate is 

```
Applies a Map operation to the entire partition of the data. The function is called once per parallel partition of the data, and the entire partition is available through the given Iterator. The number of elements that each instance of the MapPartition function sees is non deterministic and depends on the degree of parallelism of the operation.

This function is intended for operations that cannot transform individual elements, requires no grouping of elements. To transform individual elements, the use of {@code map()} and {@code flatMap()} is preferable."