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
* Karl-Friedemann Jerosch, 16.03.2022: https://www.railml.org/forum/index.php?t=msg&th=848&goto=2960&#msg_2960
* Wiki documentation:
* IS:netElement: https://wiki3.railml.org/wiki/IS:netElement
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