[SPARK-15946][MLLIB] Conversion between old/new vector columns in a DataFrame (Python)

## What changes were proposed in this pull request?

This PR implements python wrappers for #13662 to convert old/new vector columns in a DataFrame.

## How was this patch tested?

doctest in Python

cc: yanboliang

Author: Xiangrui Meng <meng@databricks.com>

Closes #13731 from mengxr/SPARK-15946.