api for grouping centers, select centers as group in "pick centers" step

for exports

Center/GroupingCenterInterface.php

@@ -0,0 +1,27 @@
+<?php
+/*
+ * 
+ */
+namespace Chill\MainBundle\Center;
+
+/**
+ * Interface to declare a groups of centers.
+ * 
+ * This interface is used to declare a groups of centers in 
+ * `Chill\MainBundle\Form\Export\PickCenterType`.
+ * 
+ */
+interface GroupingCenterInterface
+{
+    /**
+     * @return string[]
+     */
+    public function getGroups($authorizedCenters = null): array;
+    
+    /**
+     * 
+     * @param string $group
+     * @return \Chill\MainBundle\Entity\Center[]
+     */
+    public function getCentersForGroup($group);
+}