Which is more consistent with 0.7 API

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Forward port of

🩹 Fix prepare_dataset's add_svd_to_dataset function (glotaran#1522)

by making each init use named arguments

This fix relies on assert to narrow down types.
However using more uniform interfaces would be a more desirable way to solve this, but this is out of scope for a review fix.

_associated_amplitude_ -> _associated_amplitudes_
_associated_concentration_ -> _associated_concentrations_