add an interface for export which are lists

Lists does not supports aggregators.

The export manager will filter the aggregators applying on the export
depending on the interface implemented by the export: if the export implements
`Export\ListInterface`, no aggregators will be returned for this export.

Export/ListInterface.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Chill\MainBundle\Export;
+
+/**
+ * Define methods to export list.
+ * 
+ * This interface is a specification of export interface 
+ * and should be used when the export does not supports aggregators
+ * (and list does not support aggregation on their data).
+ * 
+ * When used, the `ExportManager` will not handle aggregator for this class.
+ */
+interface ListInterface extends ExportInterface
+{
+
+}