Building a Parquet Reader for Potential Integrations with Other ML Frameworks

Parquet file format is very common in some well-known frameworks like Hadoop and Spark. By enabling REEF to read parquet file, we could potentially integrate with those frameworks. Currently we want to only support data of non-nested types with a table-like property. This allows us to transform the data into formats like RDDs, etc.

A draft of ParquetReader is provided here in a PR: https://github.com/apache/reef/pull/1283