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

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)