Format and save cells.

def save(self):
        """Format and save cells."""

        # re-number cells
        self.cells = list(self.renumber())

        # add a newline to the last line if necessary
        if not self.cells[-1].endswith('\n'):
            self.cells[-1] += '\n'

        # save the rejoined the list of cells
        with open(self.filename, 'w') as file_open:
            file_open.write('\n\n'.join(self.cells))