Draft: Bugfix: enhance DC multi-slack

[vmouradian]

Please check if the PR fulfills these requirements

• [ ] The commit message follows our guidelines
• [ ] 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?

The current algorithm of slackbus selection (for most meshed at least) can select two buses connected by a non impedant branch. This causes an exception when creating equations for DC loadflow.

just comment the filter here and run MultipleSlackBusesTest#nonImpedantBranchTest to see the exception.

Currently in this branch I reproduced the bug in a unit test and added a basic draft for a possible fix.

What kind of change does this PR introduce?

What is the current behavior?

What is the new behavior (if this is a feature change)?

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

• [ ] Yes
• [ ] 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)

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