Extend <trainGroup> with parentRef

There was a request from the community to allow for hierarchical trainGroups communicated via the forum.

https://www.railml.org/forum/index.php?t=msg&goto=2556&#msg_2556

After discussing this with the developer group it was decided to implement that. This was decided after discussing the resulting potential problem that we need to modify the cardinatlities of trainGroup along with this.