Use standard-conforming detector idiom

Christoph Grüningerrequested to merge
feature/fix-use-detector-idiom into master

See also:
https://gitlab.dune-project.org/core/dune-common/-/merge_requests/1218
https://gitlab.dune-project.org/core/dune-common/-/issues/326

Change required with changes in Dune with didn't fully conform to the C++ standard interface.

Edited by Timo Koch

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