Cascading tuple support

from ParquetTupleScheme:

A Cascading Scheme that converts Parquet groups into Cascading tuples.
 
If you provide it with sourceFields, it will selectively materialize only the columns for those fields. The names must match the names in the Parquet schema.

If you do not provide sourceFields, or use Fields.ALL or Fields.UNKNOWN, it will create one from the Parquet schema.

Currently, only primitive types are supported. TODO: allow nested fields in the Parquet schema to be flattened to a top-level field in the Cascading tuple.