Three-winding transformers creation

[colinepiloquet]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Feature

What is the new behavior (if this is a feature change)? It is now possible to create a three-winding transformer. Tap changer can also be created on three-winding transformers. Operational limits are already available.

Does this PR introduce a breaking change or deprecate an API?

• [ ] Yes
• [x] No

[EtienneLt]

Could you add the documentation in docs/ ?

[colinepiloquet]

Could you add the documentation in docs/ ?

Yes I will, actually I have to work on a few more things too, creation of tap changers, limits.