Merged SV has dangling references

[Haigutus]

It seems that if there is mix on NodeBraekre and BusBranch models in merge, eg models modelled with Connectivity Nodes and models Topological Nodes, powsybl recreates TP instance and refers from SV to the new TP instance, thus references to original one are dangling.

Possible fixes:

1. Check if issue is present in newest pypowsybl
2. Test with different import settings
3. Update exported SV

In case of 3.

1. Export also TP to CGMES from pypowsybl
2. Find links to CN
3. Find original TP
4. Update SV with original TP

[Haigutus]

1. Done - did not help
2. Starting to test