Redundant references in railML 3.x topology
## Description ##
There is a redundancy for modelling the linkage between `<netElement>` and `<netRelation>`. The link from the `<netRelation>` to the `<netElement>` is mandatory, while the link from the `<netElement>` to the `<netRelation>` is optional.
### Background ###
This way of modelling results from RailTopoModel.
### Links ###
* Forum discussion:
* Raphael Gaudy, 15.09.2021: [https://www.railml.org/forum/index.php?t=msg&th=828&goto=2827&#msg_2827](https://www.railml.org/forum/index.php?t=msg&th=828&goto=2827&#msg_2827)
* Git issues:
* #10
* https://development.railml.org/railml/version3/-/issues/481
* Wiki documentation:
* CO:relation: [https://wiki3.railml.org/wiki/CO:relation](https://wiki3.railml.org/wiki/CO:relation)
## Proposed solution railML 3.2 ##
The optional repeatable child element `<relation>` in element `<netElement>` shall be marked DEPRECATED. The link between `<netElement>` and `<netRelation>` shall only be modelled using the (mandatory) child elements `<elementA>` and `<elementB>` in `<netRelation>`.
issue