Z1: Structure change to CSV files

In response to user feedback and to meet accessibility requirements, the format of the new compressed CSV files has changed since their June introduction. Specifically, these files now show additional precision and now only include series mnemonics and data values. For each CSV file, a corresponding data dictionary text file will be provided with additional series-level information. To help users transition to the new file format, updated compressed PRN files will also be posted with the September 16, 2016 release.