class_docstr: This class handles iteration events to store the current performance as
  an attribute of the sender (i.e. application).
code: def __init__(self, **_unused):
      ITER_FINISHED.connect(self.update_performance_history)
  
imports: import tensorflow
  from niftynet.engine.application_variables import CONSOLE
  from niftynet.engine.signal import ITER_FINISHED
label_desc: Use image node_id 1 to create a new PerformanceLogger object from inherited base classes: object with example: obj = PerformanceLogger()