docs: def get_ioos_def(self, ident, elem_type, ont):
          """Gets a definition given an identifier and where to search for it"""
          if elem_type == "identifier":
              getter_fn = self.system.get_identifiers_by_name
          elif elem_type == "classifier":
              getter_fn = self.system.get_classifiers_by_name
          else:
              raise ValueError("Unknown element type '{}'".format(elem_type))
          return DescribeSensor.get_named_by_definition(
              getter_fn(ident), urljoin(ont, ident)
          )