add getter/setter for extractor_tasks of BaseSession

[bimac]

Choosing the correct extractor tasks in a task init method causes problems as the superclass init already writes the extractor tasks to the experiment_description property which then must be modified by the subclass init:
https://github.com/int-brain-lab/project_extraction/blob/b68545e921c2a93e14eeae652921133d21bc73c4/iblrig_custom_tasks/samuel_cuedBiasedChoiceWorld/task.py#L25-L28

This problem is somewhat mitigated as tasks can be called XBpod, XTimeline, XNidq, etc. in which case calling the task X in the experiment description will cause the pipeline to choose the correct class based on the sync. For irregularly-named classes, however, the name should be defined explicitly so for this to be done dynamically we should have a getter and setter for the extractor_tasks property that can be overloaded by subclasses.