Cleans the values in each cell. Calls either the user provided clean_value, or the
    class defined clean value.

def clean_values(self):
    """Cleans the values in each cell. Calls either the user provided clean_value, or the
    class defined clean value.
    """
    for row in self.rows:
      for index, cell in enumerate(row):
        self.set_cell_value(row, index, self.clean_value(self.get_cell_value(cell)))
    return self