Returns the col at index `colnum` or name `colnum`.

def col(self, col_name_or_num):
        """
        Returns the col at index `colnum` or name `colnum`.
        """
        if isinstance(col_name_or_num, basestring):
            return self[col_name_or_num]
        elif isinstance(col_name_or_num, (int, long)):
            if col_name_or_num > len(self.fields):
                raise IndexError("Invalid column index `%s` for DataTable" %
                                 col_name_or_num)
            return self.__data[self.fields[col_name_or_num]]