Return selected sequence reference from GetInitialGuess and remove copy-based test

[sinshu]

Motivation

• Return the selected initial barycenter sequence by reference from GetInitialGuess to avoid unnecessary copying and to match the updated documentation.
• Remove the test which asserted that GetInitialGuess returned a copy because the intended behavior is now to return the original sequence.

Description

• Changed DtwBarycenterAveraging.GetInitialGuess to return sourceSequences[bestIndex] instead of creating and returning a copied array of Vec<double> instances.
• Updated the XML doc comment to state that the selected sequence is returned (not a copy).
• Removed the GetInitialGuessSelectsMedoidWithSmallestTotalDtwDistance test from DtwBarycenterAveragingTests which asserted the returned sequence was not the same instance as the source.

Testing

• Ran the NumFlatTest unit test suite with dotnet test, and the tests passed.
• Verified that FitMatchesTslearnDocumentedExampleWithEqualLengthSeries, FitKeepsInitialGuessLengthForDifferentLengthSeries, UpdateDoesNotIncreaseObjectiveCost, and SingleSourceSequenceIsReturnedUnchangedByFit all completed successfully.

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Codex Task