Update aggs.py

add class support weighted average aggregation. https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-weight-avg-aggregation.html

Author: truongvan

elasticsearch_dsl/aggs.py

@@ -228,7 +228,10 @@ def result(self, search, data):
 
 class Avg(Agg):
     name = 'avg'
-
+    
+class WeightedAvg(Agg):
+    name = 'weighted_avg'
+    
 class Cardinality(Agg):
     name = 'cardinality'