code: def get_directory(self, identifier):
          
          return os.path.join(
              os.path.join(self.directory, identifier[:2]),
              identifier
          )
docs: Implements the policy for naming directories for image objects. Image
          object directories are name by their identifier. In addition, these
          directories are grouped in parent directories named by the first two
          characters of the identifier. The aim is to avoid having too many
          sub-folders in a single directory.
  
          Parameters
          ----------
          identifier : string
              Unique object identifier
  
          Returns
          -------
          string
              Path to image objects data directory