Voltage and angle in 3wt star buses and boundary nodes

[mathbagu]

• Do you want to request a feature or report a bug?
  Feature

• What is the current behavior?

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

• What is the expected behavior?
  Backport of :

• Compute star bus voltages of threeWindingsTransformers and boundary bus voltages of dangling lines powsybl-core#1375
• Cgmes. Voltage and angle in 3wt star buses and boundary nodes powsybl-core#1811

• What is the motivation / use case for changing the behavior?

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

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