schema CHANGE add lysp_node_groupings()

To get the groupings member from some of the schema nodes.
diff --git a/src/tree_schema_helpers.c b/src/tree_schema_helpers.c
index 0146f43..f9059fe 100644
--- a/src/tree_schema_helpers.c
+++ b/src/tree_schema_helpers.c
@@ -921,6 +921,27 @@
     }
 }
 
+API const struct lysp_grp *
+lysp_node_groupings(const struct lysp_node *node)
+{
+    switch (node->nodetype) {
+    case LYS_CONTAINER:
+        return ((struct lysp_node_container*)node)->groupings;
+    case LYS_LIST:
+        return ((struct lysp_node_list*)node)->groupings;
+    case LYS_GROUPING:
+        return ((struct lysp_grp*)node)->groupings;
+    case LYS_ACTION:
+        return ((struct lysp_action*)node)->groupings;
+    case LYS_INOUT:
+        return ((struct lysp_action_inout*)node)->groupings;
+    case LYS_NOTIF:
+        return ((struct lysp_notif*)node)->groupings;
+    default:
+        return NULL;
+    }
+}
+
 struct lysp_action **
 lysp_node_actions_p(struct lysp_node *node)
 {