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

for exports

Resources/views/Export/new_centers_step.html.twig

@@ -35,6 +35,15 @@
 
     {{ form_widget(form.centers.c) }}
     
+    {% if form.centers.children.g is defined %}
+        
+        <h3>{{ 'Pick aggregated centers'|trans }}</h3>
+        
+        {% for f in form.centers.children.g.children %}
+        {{ form_row(f) }}
+        {% endfor %}
+    {% endif %}
+    
     <p>{{ form_widget(form.submit, { 'attr' : { 'class' : 'sc-button btn-action bt-create' }, 'label' : 'Go to export options' } ) }}</p>
     {{ form_end(form) }}