question_series in Base

[albertas-jn]

Think about if question_series should be defined in Base. It adds question series to experiments that don't need to ask any questions (Toontjehoger). Otherwise, assign question_series=[] in extended classes.

[BeritJanssen]

I included a commit to remove the question_series from base.py while working on #847. I need to go over all rules files for that issue, in order to check which session methods they call, and it kind of itched in my fingers to go ahead and remove it.

[albertas-jn]

OK, but it should not be completely removed, but set to an empty list. I'd also keep the current question_series as a comment/doc in the file just as an example how it can be done if base.py is ever used as a template.