Optimized Conll Reader

[albertoandreottiATgmail]

Description

This change adds a new capability to the CoNLL reader, allowing it to read multiple CoNLL files at the same time into a single Dataframe. Example,

df = CoNLL().readDataset(spark, './path/to/conlls/*', partitions=12)

The difference with previous behavior is the path ending in an *. The original mechanism continues to work as before.
Two additional params that work only for the multi file situation,
partitions : minimum number of partitions used to create the Dataframe. Defaults to 8.
storage_level : pyspark.StorageLevel for the Dataframe. Defaults to pyspark.StorageLevel.DISK_ONLY.

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code improvements with no or little impact
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING page.
• I have added tests to cover my changes.
• All new and existing tests passed.