[FEATURE] merge nodes connected by link into one bus in mathematical model

[TonyXiang8787]

Currently the link is modeled as a small impedance. This can cause numerical issues.

It is proposed to merge the nodes connected by link into one single bus.

Acceptance criteria

• Nodes in topology that are connected by links are merged automatically.
• To be discussed: do we still calculate the power flow in links?
• Results extrapolated to original nodes
• Unit tests
• Validation tests [Merge nodes by links validation case #1507]

Full release is blocked by #1418 [DONE]

[scud-soptim]

Good morning, does that mean you calculate the current of the link via KCL?”