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?
Please check if the PR fulfills these requirements
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?