Get more columns or rows (based on axis).

def fetch_more(self, rows=False, columns=False):
        """Get more columns or rows (based on axis)."""
        if  self.axis == 1 and self.total_rows > self.rows_loaded:
            reminder = self.total_rows - self.rows_loaded
            items_to_fetch = min(reminder, ROWS_TO_LOAD)
            self.beginInsertRows(QModelIndex(), self.rows_loaded,
                                 self.rows_loaded + items_to_fetch - 1)
            self.rows_loaded += items_to_fetch
            self.endInsertRows()
        if self.axis == 0 and self.total_cols > self.cols_loaded:
            reminder = self.total_cols - self.cols_loaded
            items_to_fetch = min(reminder, COLS_TO_LOAD)
            self.beginInsertColumns(QModelIndex(), self.cols_loaded,
                                    self.cols_loaded + items_to_fetch - 1)
            self.cols_loaded += items_to_fetch
            self.endInsertColumns()