WIP: add support for receiving single elements of lists

- resolved weird bug, that causes errors when nonterminal A (in the singleList-test) has no children.
- add new testcase for different structure/variants of some node to be sent/received
4 jobs for 21-allow-connection-endpoints-for-list-nonterminals in 10 minutes and 57 seconds (queued for 7 seconds)