feat: add _drop_lowest() to VAFE

Drops lowest n% (default:33%) of scdet scores, since it scores every
frame

Python being what it is, this could be a single line in another method
but pulling it out into another function:

 - makes explicit what we are doing and lets us document why
 - makes for easier testing

pipeline/feature_extractors.py

@@ -282,6 +282,11 @@ class VideoActivityFeatureExtractor(FeatureExtractor):
                                 )
         return scores
 
+    def _drop_lowest(self, scores, percent=33):
+        """Drop the lowest n% scores from the list"""
+        scores = sorted(scores, key=lambda x: x[1], reverse=True)
+        return scores[:int(len(scores) * (percent / 100))]
+
     def setup(self):
         pass