Method constructing a series from a given question ID should set the name of the index column

I would like to suggest to set the name of the index column series.index.name = "id" in method as_series in class question like this:

    def as_series(self) -> Series:
        """
        Obtain the answers to this question as a pandas.Series.

        The series' index are the participant IDs, while data for the
        indices are the respective answers.

        The series will be named with the question's full ID.

        Returns:
            A pandas.Series representing the answers for each participant
        """
        series = Series(self._answers)
        series.name = self.full_id
        series.index.name = "id"
        return series

A resulting DataFrame would then look something like this:

   Q002/SQ001  Q003/SQ001
id                       
1     Option1         123
2     Option2         456
3     Option3         789

Relates to #111 (closed).