Let unique() also return the indices

[Markus-Goetz]

Feature functionality
NumPy provides a return_indices flag for the unique() call that returns the indices of the original array. Provide the additional functionality.

[ClaudiaComito]

Addressed in #749

[ClaudiaComito]

Still addressed in #749, will get there after #938

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Reviewed within #1109

[github-actions]

Branch features/493-Let_unique_also_return_the_indices created!