scan with list of variable count_times
For a variable time scan in the Exafs region we need to pass a list of count times.
chain = DEFAULT_CHAIN.get(scan_info, counter_args, top_master=VariableStepTriggerMaster(motor, positions), timer_master=SoftwareListTimerMaster(count_times, npoints=len(count_times), sleep_time=sleep_time))
The default timer_master is None which will leave the timer to be SoftwareTimerMaster.