CDK-496: Added the type parameter to the Datasets API

I also went through and extended the use of the type parameter down to at least Avro files
and CSV files. In order to support Parquet and HBase, I think more work needs to be done
on the underlying stores. But if I missed something, I can get those in before the merge.
Naming: other classes in this package are Util, not Utils. Also, DataUtil is very general, how about DataModelUtil, or could these methods go in SchemaUtil?