Extend <netElement> with <designator> child element

Description: The topology element <netElement> shall be extended with a repeatable optional child element <designator>.

Links:

• Minutes ETCS use case working group meeting 18.10.2021: [https://www.railml.org/en/event-reader/railml-is-etcs-telco-2021-10-18.html]
• Forum:
  • Christian Rahmig, 17.01.2022: [https://www.railml.org/forum/index.php?t=msg&goto=2879&#msg_2879]
• Git issues:
  • #476 (moved)
  • railtopomodel#1 (closed)
• Wiki documentation:
  • IS:netElement: [https://wiki3.railml.org/wiki/IS:netElement]