code: def cached(
              cls,
              release=MAX_ENSEMBL_RELEASE,
              species=human,
              server=ENSEMBL_FTP_SERVER):
          
          init_args_tuple = cls.normalize_init_values(release, species, server)
          if init_args_tuple in cls._genome_cache:
              genome = cls._genome_cache[init_args_tuple]
          else:
              genome = cls._genome_cache[init_args_tuple] = cls(*init_args_tuple)
          return genome
docs: Construct EnsemblRelease if it's never been made before, otherwise
          return an old instance.