Algorithm to average on scores

Hi,

This MR is to develop and add a new algorithm to calculate the final scores by calculating the average of scores over different reference samples of each user. The current version does the averaging on the feature values (in the enroll() function).

ping @tiago.pereira

bob/bio/base/pipelines/vanilla_biometrics/abstract_classes.py

@@ -11,7 +11,7 @@ import logging
 logger = logging.getLogger(__name__)
 
 
-def average_scores(scores):
+def reduce_scores(scores, fn=np.max):
     """
     Given a :any:`numpy.ndarray` coming from multiple probes,
     average them.
@@ -29,7 +29,7 @@ def average_scores(scores):
 
     # First we have to average w.r.t tp individual samples, than between samples
 
-    return np.mean(np.array([np.mean(x, axis=1) for x in scores]), axis=0)
+    return fn(np.array([fn(x, axis=1) for x in scores]), axis=0)
 
 
 class BioAlgorithm(metaclass=ABCMeta):
@@ -46,7 +46,7 @@ class BioAlgorithm(metaclass=ABCMeta):
 
     """
 
-    def __init__(self, score_reduction_operation=average_scores, **kwargs):
+    def __init__(self, score_reduction_operation=reduce_scores, **kwargs):
         self.stacked_biometric_references = None
         self.score_reduction_operation = score_reduction_operation