code: def _file_path(cls, checksum, download_dir):
          
          checksum = checksum.lower()
          file_prefix = '_'.join(['files'] + list(map(str, cls.DIRECTORY_NAME_LENGTHS)))
          path_pieces = [download_dir, '.hca', 'v2', file_prefix]
          checksum_index = 0
          assert(sum(cls.DIRECTORY_NAME_LENGTHS) <= len(checksum))
          for prefix_length in cls.DIRECTORY_NAME_LENGTHS:
              path_pieces.append(checksum[checksum_index:(checksum_index + prefix_length)])
              checksum_index += prefix_length
          path_pieces.append(checksum)
          return os.path.join(*path_pieces)
docs: returns a file's relative local path based on the nesting parameters and the files hash
          :param checksum: a string checksum
          :param download_dir: root directory for filestore
          :return: relative Path object