Add ability to write data to Rfiles instead of Parquet files

[gaffer01]

Add a table property that controls whether data is written to Rfiles or Parquet files. Add a writer and a reader that uses Rfiles. This will require concatenating fields (e.g. if there are two row key fields then their values will need to be concatenated into the row key in the Rfile). There is code in Gaffer to do this.

This issue should be done after #26.