Allow chaining of aggregation

src/Aggregation/AbstractAggregation.php

@@ -76,6 +76,8 @@ abstract class AbstractAggregation implements BuilderInterface
      * Adds a sub-aggregation.
      *
      * @param AbstractAggregation $abstractAggregation
+     *
+     * @return $this
      */
     public function addAggregation(AbstractAggregation $abstractAggregation)
     {
@@ -84,6 +86,8 @@ abstract class AbstractAggregation implements BuilderInterface
         }
 
         $this->aggregations->add($abstractAggregation);
+        
+        return $this;
     }
 
     /**