update export & aggregator & filter to new api + add tests

Export/Export/CountActivity.php

@@ -102,10 +102,13 @@ class CountActivity implements ExportInterface
             throw new \LogicException("the key $key is not used by this export");
         }
         
-        $labels = array_combine($values, $values);
-        $labels['_header'] = 'Number of activities';
-        
-        return $labels;
+        return function($value) {
+            return $value === '_header' ? 
+                    'Number of activities'
+                    :
+                    $value
+                    ;
+        };
     }
 
     public function getQueryKeys($data)