Add columns to table if they are not already there.

    Args:
      table: table name as a string
      columns: an iterable of column names

def AddTableColumns(self, table, columns):
    """Add columns to table if they are not already there.

    Args:
      table: table name as a string
      columns: an iterable of column names"""
    table_columns = self._table_columns.setdefault(table, [])
    for attr in columns:
      if attr not in table_columns:
        table_columns.append(attr)