Add basic application framework and example application to show interaction between events an NBI

Depends on:

• !340 (merged)
• !344 (merged)
• !349 (merged)
• !342 (merged)

controller/nucleus/memoryPndStore.go

@@ -45,7 +45,7 @@ func (t *MemoryPndStore) Delete(item networkdomain.NetworkDomain) error {
 func (t *MemoryPndStore) Get(query store.Query) (networkdomain.NetworkDomain, error) {
 	item, ok := t.Store[query.ID]
 	if !ok {
-		return nil, nil
+		return nil, &nerrors.ErrNotFound{ID: query.ID}
 	}
 
 	return item, nil