Since Java and Scala both have access to iterate over partitions via the "toLocalIterator" function, python should also have that same ability.

Author: Michael Nazario <mnazario@palantir.com>

Closes #4237 from mnazario/feature/toLocalIterator and squashes the following commits:

1c58526 [Michael Nazario] Fix documentation off by one error
0cdc8f8 [Michael Nazario] Add toLocalIterator to PySpark