docs: def rtype_to_model(rtype):
      """ Return a model class object given a string resource type
  
      :param rtype:
          string resource type
      :return:
          model class object
      :raise:
          ValueError
      """
  
      models = goldman.config.MODELS
      for model in models:
          if rtype.lower() == model.RTYPE.lower():
              return model
      raise ValueError('%s resource type not registered' % rtype)