docs: def add_row(self, list_or_dict, key=None):
          """ Adds a list or dict as a row in the FVM data structure
  
          :param str key: key used when rows is a dict rather than an array
          :param list_or_dict: a feature list or dict
          """
          if isinstance(list_or_dict, list):
              self._add_list_row(list_or_dict, key)
          else:
              self._add_dict_row(list_or_dict, key)