docs: def load_mode_validator(obs_mode, node):
      """Load observing mode validator"""
  
      nval = node.get('validator')
  
      if nval is None:
          pass
      elif isinstance(nval, str):
          # load function
          obs_mode.validator = import_object(nval)
      else:
          raise TypeError('validator must be None or a string')
  
      return obs_mode