Add a primitive that transforms key-value pair into columns

Write a primitive that transforms key-value pair in a table into columns.

The primitive is based on comparing column names between two tables and detecting whether the table contains key/value columns.

This primitive is tested against the world-development-dataset ( https://www.kaggle.com/worldbank/world-development-indicators ). There is also unit-test code in /tests/test_key_value_to_column.py.