twoqbcal: raise understandable exection if None is passed as task list in the init of MultiTaskingExperiment, DynamicPhase, and Chevron.
twoqbcal: raise understandable exection if None is passed as task list
in the init of MultiTaskingExperiment, DynamicPhase, and Chevron.

This is to avoid that an cryptic exception is raised when trying to
iterate over the task_list.

(Not needed in CPhase, which directly passes the task_list to the super
init, so that the exection would be triggeren in MultiTaskingExperiment.)

QudevETH/PycQED_py3