IIDM ThreeWindingsTransformer RatedU0 modification

[marqueslanauja]

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)
• [ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

What kind of change does this PR introduce?

Feature

What is the current behavior?

What is the new behavior (if this is a feature change)? Allows to modify the ratedU0 attribute of a threeWindingsTransformer. As the structural ratio of the three legs must be preserved, the ratedU of the three legs must be adjusted.

To set the ratedU0 attribute I added a new method, setRatedU0, in the threeWindingsTransformer interface.

I feel more comfortable with the previous interface where ratedU0 can only be set when the transformer is created.

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

• [ ] Yes
• [x] No

If yes, please check if the following requirements are fulfilled

• [ ] The Breaking Change or Deprecated label has been added
• [ ] The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information: