def aggregate(self, accumulatedError, historyBuffer, steps):
Updates the final aggregated score error given the prediction and the ground 
truth.

:param accumulatedError: The total accumulated score from the previous
       predictions (possibly over some finite window)

:param historyBuffer: A buffer of the last <self.window> ground truth values
       that have been observed. If ``historyBuffer`` = None,  it means that 
       no history is being kept.

:param steps: (int) The total number of (groundTruth, prediction) pairs that 
       have been passed to the metric. This does not include pairs where 
       ``groundTruth = SENTINEL_VALUE_FOR_MISSING_DATA``

:returns: The new aggregate (final) error measure.