Trace containing 40000012 event types gets "UserWarning: Unable to chop to ROI" with --chop-to-roi 

Running pypop-preprocess on a trace with tracing on / off events and --chop-to-roi gets a warning 

UserWarning: Unable to chop to ROI: (No valid Extrae ON-OFF bracket in trace) - Continuing without chopping
  warn("Unable to chop to ROI: ({}) - Continuing without chopping".format(err))

