code: def _normalize_files(item, fc_dir=None):
      
      files = item.get("files")
      if files:
          if isinstance(files, six.string_types):
              files = [files]
          fastq_dir = flowcell.get_fastq_dir(fc_dir) if fc_dir else os.getcwd()
          files = [_file_to_abs(x, [os.getcwd(), fc_dir, fastq_dir]) for x in files]
          files = [x for x in files if x]
          _sanity_check_files(item, files)
          item["files"] = files
      return item
docs: Ensure the files argument is a list of absolute file names.
      Handles BAM, single and paired end fastq, as well as split inputs.