PSS/E import - support boundary trafo between area

[pjanecek0]

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• Tests for the changes have been added (for bug fixes / features)
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Does this PR already have an issue describing the problem?

#3390

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#3390

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[marqueslanauja]

I agree with the changes.

Maybe combine the two unit tests into a single test case. We try to reduce the number of files and minimize the overall size used in testing.