Make @type optional

The attribute <track>@type shall become optional.

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 discussion:
  • Christian Rahmig, 17.01.2022: https://www.railml.org/forum/index.php?t=msg&goto=2880&#msg_2880
• Git issues:
  • #475 (closed)
• Wiki documentation:
  • IS:track: https://wiki3.railml.org/wiki/IS:track