WIP: Download demux issue 533

So far, this only provides the PreprocessedData summary.
@biocore/qiita-dev I defined this method because `BaseData.get_filepaths` didn't return all of the data that I wanted (namely, the english language description of the filepath type).  Would it be better just to modify `get_filepaths` to return that information, and then fix the rest of the codebase to expect this API change?