Check how associations are handled during FAIR data station merging

[stuzart]

Currently child associations are removed and then rebuilt, e.g.

https://github.com/seek4science/seek/blob/main/lib/seek/fair_data_station/writer.rb#L149

but this could have implications for partial merges, where the template only contains information items to be updated, or where items have been associated through the UI.
It's not currently clear how easy this would be to change, and if difficult how much of a priority it is. Needs some test cases building and playing around with different solutions.